Weird Problems with Starving Venom Fusion Dragon

Question

My friend and I were dueling, and he summoned Starving Venom Fusion Dragon, and Starving Venom Fusion Dragon can copy an opponent's card's effect. So he copied my Cyber Dragon Infinity's effect. Cyber Dragon Infinity's effect allows you to target one face-up monster on the field and attach it to Cyber Dragon Infinity as material.

However, my friend used Starving Venom Fusion Dragon's newly gained effect to add my Cyber Dragon Infinity as material to his Starving Venom Fusion Dragon. My first question is can a Fusion monster ever have Xyz materials?

The next turn, my friend used Starving Venom Fusion Dragon to copy my Cyber Shadow Gardna's effect. Cyber Shadow Gardna is a Trap Monster and part of it's effect says that during the End Phase, set this card back. So my second question is, can a monster be set like a trap?

Starving Venom Fusion Dragon's Effect:

2 DARK monsters on the field, except Tokens

If this card is Fusion Summoned: You can make this card gain ATK equal to 1 opponent's Special Summoned monster, until the end of this turn. Once per turn: You can target 1 Level 5 or higher monster your opponent controls; until the End Phase, this card's name becomes that monster's original name, and replace this effect with that monster's original effects. If this Fusion Summoned card is destroyed: You can destroy all your opponent's Special Summoned monsters.

Cyber Dragon Infinity's Effect:

3 Level 6 LIGHT Machine monsters

Once per turn, you can also Xyz Summon "Cyber Dragon Infinity" by using "Cyber Dragon Nova" you control as material. (Transfer its materials to this card.) Gains 200 ATK for each material attached to it. Once per turn: You can target 1 face-up Attack Position monster on the field; attach it to this card as a face-up material. Once per turn, when a card or effect is activated (Quick Effect): You can detach 1 material from this card; negate the activation, and if you do, destroy it.

Cyber Shadow Gardna's Effect (Trap):

Activate only during your opponent''s Main Phase. After activation, Special Summon this card, it is treated as an Effect Monster Card (Machine-Type/EARTH/Level 4/ATK ?/DEF ?). (This card is still treated as a Trap Card.) If this card is selected as an attack target, the ATK and DEF of this card become the ATK and DEF of the attacking monster. This card is Set in its Spell & Trap Card Zone during your opponent''s End Phase.

Answer 1

You asked two questions, but the real answer why your friend couldn't do that is:

• Starving Venom Fusion Dragon explicitly says that it can only target Lvl 5 or higher monsters.

Nor Gardna not your XYZ apply to that, as Gardna is lvl 4 and Cyber Dragon does not have a level (as it is an XYZ monster, it has Rank instead). Although the XYZ is Rank 6, Rank is not the same as Level.

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To answer your other questions, only XYZ monsters can have XYZ materials. That means that such effect couldn't have been activated even in this erroneous play scenario.

Now, in YuGiOh some monsters can be set as Spell/Trap cards, such as the Artifact Archetype. However, they specifically indicate that you can set them in such way. Finally, even supposing you copied Gardna's effect and supposing you Starving Venom as Spell/Trap, the effect copied explicitly reads at the end (emphasis mine):

[...] This card is Set in its Spell & Trap Card Zone during your opponent''s End Phase.

So it is specifying it has to be set in its corresponding Spell/Trap zone. As Starving Venom does not have one it is logical that the effect can't be activated by failing to meet the requirement.

Finally, as mentioned in comments, Starving Venom copy effect ends at the end phase. Thus, it will never reach the opponent's end phase for the setting effect to take place.