A: As in this scenario, if there are no available Monster Zones to Special Summon in when resolving "Monster Reborn Reborn", its effect is not applied. (In this case, all 3 targeted monsters remain in the Graveyard.)[2]
A: When resolving the effect of "Monster Reborn Reborn", if one of the three targeted monsters is no longer in the Graveyard, the effect is applied using the remaining 2 monsters (in this scenario, "X-Head Cannon" and "Z-Metal Tank"). The monster chosen by your opponent is Special Summoned, and the remaining monster is banished.[4]
A: If the effect of "Imperial Iron Wall" is applying when "Monster Reborn Reborn" resolves, although your opponent still chooses 1 of the targeted monsters for you to Special Summon, the part of the effect that banishes the remaining monsters is not applied, and those monsters remain in the Graveyard. Also, if the effect of "Imperial Iron Wall" is already applying, you cannot activate "Monster Reborn Reborn".[5]