If I have Pramikon, Sky Rampart out and attach a Helm of the Host to it, would it allow a new copy to enter, allowing me to choose the opposite direction from my original Pramikon? If so would that mean that no one can conduct any attacks in any direction?
If you look at the rulings on the gather page for Pramikon, Sky Rampart that is the case.
If two Pramikons on the battlefield disagree on which direction three or more players must attack, players can’t attack at all.
Each one says that players may not attack any player other than the one in the given direction, so if one copy says that players may not attack anyone other than the player to their right, and another says that no one may attack anyone other the player to their left, then no one may attack any player that is not both to their left and to their right. Each copy provides an entirely restrictive condition, and does not permit anything that would otherwise be disallowed. That is, "players may only do X" in MtG never means "players may do X", it only ever means "players may not do other than X". In a game with more than two players, no player will be both to the right and to the left, and so no one can be attacked. In a two player game, or in a multiplayer game in which all but two players have been eliminated, each player is both to the right and the left of the other, and so both players can attack each other. The wording of the ruling on Gatherer seems to claim otherwise, but I think that the author was assuming a multiplayer game. I don't see any way to read the text of Pramikon, Sky Rampart as having any effect in a two player game.
See also Cranial Insertion
With two Pramikons on the battlefield naming different directions (or a Pramikon and a Mystic Barrier, etc.) then players can't attack players or planeswalkers unless they're (controlled by) the opponent who is both immediately to the left and immediately to the right of that player. In a multiplayer game, this player can't be the same unless there is only one opponent remaining in the game. Once there is one opponent remaining, then left equals right and the Sky Rampart's domination of the battlefield is broken allowing creatures to attack once again.
and the Gatherer ruling for Mystic Barrier
If two Mystic Barriers are on the battlefield with two different chosen directions, you may not attack a player or planeswalker unless that player or the controller of that planeswalker is the opponent seated nearest you in both directions. (This happens most often in two-player games.)
Also note that it say "nearest opponent". The Gatherer page for Mystic Barrier says
In some formats, you may need to disregard teammates sitting between you and the opponent seated nearest you in the chosen direction. In formats like Emperor, other rules may prohibit you from attacking that player.
So two of these cards be a powerful combination in Archenemy, as it would prohibit the Archenemy from attacking anyone, while the other players would be free to attack the Archenemy.
If WotC decided to make Phasing apply to players, then having all but two players Phased should also allow them to attack each other.