During Warp World's resolution, you have to put enchantment creatures on the battlefield as creatures. However, during Warp World's resolution, no triggered abilities enter the stack either, so for Murderous Redcap's ability, this specific order doesn't matter. Ultimately, you can't prevent Grenzo from fetching the Faithless Looting.

Warp World gives clear instructions on the order in which to place permanent cards on the battlefield: you put all artifact, creature, and land cards down in any order, then enchantments (so that auras have something to attach to), and the rest to the bottom of the library. Enchantment creatures are creatures, so you put them down together with the other non-enchantments.

During the resolution, ETB effects trigger as normal. However, they do not enter the stack yet, so you don't to choose any targets yet:

117.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack.

Therefore, by the time you have to choose a target for Murderous Redcap, all permanents will be on the battlefield to choose a target from, so it would not matter whether you put down enchantment creatures as creatures or enchantments. Consequently, it also does not matter whether, in the non-enchantment part of WW, you put down the Redcap or the Grenzo first.

When the Warp World has finished resolving, the order of triggered abilities is determined by priority: The player with priority orders their abilities first, then all other players in turn order.

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose. [..]

In your specific case, that would mean player 1 puts Grenzo's ability on the stack, then at some point player 3 puts Redcap's ability on the stack. Redcap kills the Grenzo, but Grenzo's ability will still resolve and use the last known information from Grenzo, which means the Faithless Looting will be put into the graveyard.

During Warp World's resolution, you have to put enchantment creatures on the battlefield as creatures. However, during Warp World's resolution, no triggered abilities enter the stack either, so for Murderous Redcap's ability, this specific order doesn't matter. Ultimately, you can't prevent Grenzo from fetching the Faithless Looting.

Warp World gives clear instructions on the order in which to place permanent cards on the battlefield: you put all artifact, creature, and land cards down in any order, then enchantments (so that auras have something to attach to), and the rest to the bottom of the library. Enchantment creatures are creatures, so you put them down together with the other non-enchantments.

During the resolution, ETB effects trigger as normal. However, they do not enter the stack yet, so you don't to choose any targets yet:

117.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack.

Therefore, by the time you have to choose a target for Murderous Redcap, all permanents will be on the battlefield to choose a target from, so it would not matter whether you put down enchantment creatures as creatures or enchantments. Consequently, it also does not matter whether, in the non-enchantment part of WW, you put down the Redcap or the Grenzo first.

When the Warp World has finished resolving, the order of triggered abilities is determined by priority: The player with priority orders their abilities first, then all other players in turn order.

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose. [..]

That means that only when Warp World has finished resolving will the abilities go on the stack, and player 1 has an opportunity to activate Grenzo before Redcap's ability resolves. You have no way of preventing that in your scenario.

During Warp World's resolution, you have to put enchantment creatures on the battlefield as creatures. However, during Warp World's resolution, no triggered abilities enter the stack either, so for Murderous Redcap's ability, this specific order doesn't matter.

Warp World gives clear instructions on the order in which to place permanent cards on the battlefield: you put all artifact, creature, and land cards down in any order, then enchantments (so that auras have something to attach to), and the rest to the bottom of the library. Enchantment creatures are creatures, so you put them down together with the other non-enchantments.

During the resolution, ETB effects trigger as normal. However, they do not enter the stack yet, so you don't to choose any targets yet:

117.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack.

Therefore, by the time you have to choose a target for Murderous Redcap, all permanents will be on the battlefield to choose a target from, so it would not matter whether you put down enchantment creatures as creatures or enchantments. Consequently, it also does not matter whether, in the non-enchantment part of WW, you put down the Redcap or the Grenzo first.

When the Warp World has finished resolving, the order of triggered abilities is determined by priority: The player with priority orders their abilities first, then all other players in turn order.

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose. [..]

In your specific case, that would mean player 1 puts Grenzo's ability on the stack, then at some point player 3 puts Redcap's ability on the stack. Redcap kills the Grenzo, but Grenzo's ability will still resolve and use the last known information from Grenzo, which means the Simulacrum will return.

During Warp World's resolution, you have to put enchantment creatures on the battlefield as creatures. However, during Warp World's resolution, no triggered abilities enter the stack either, so for Murderous Redcap's ability, this specific order doesn't matter. Ultimately, you can't prevent Grenzo from fetching the Faithless Looting.

Warp World gives clear instructions on the order in which to place permanent cards on the battlefield: you put all artifact, creature, and land cards down in any order, then enchantments (so that auras have something to attach to), and the rest to the bottom of the library. Enchantment creatures are creatures, so you put them down together with the other non-enchantments.

During the resolution, ETB effects trigger as normal. However, they do not enter the stack yet, so you don't to choose any targets yet:

117.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack.

Therefore, by the time you have to choose a target for Murderous Redcap, all permanents will be on the battlefield to choose a target from, so it would not matter whether you put down enchantment creatures as creatures or enchantments. Consequently, it also does not matter whether, in the non-enchantment part of WW, you put down the Redcap or the Grenzo first.

When the Warp World has finished resolving, the order of triggered abilities is determined by priority: The player with priority orders their abilities first, then all other players in turn order.

603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose. [..]

In your specific case, that would mean player 1 puts Grenzo's ability on the stack, then at some point player 3 puts Redcap's ability on the stack. Redcap kills the Grenzo, but Grenzo's ability will still resolve and use the last known information from Grenzo, which means the Faithless Looting will be put into the graveyard.