This is probably in the TR, and if it is, I'm sorry for asking.

When you change the zone of an attack, after that attack resolves, does its zone remain changed?

For example, say I play Ichi, change it's zone with bitter rivals to high, then it resolves. Will the zone remain as a high, so that, for example, I play Midnight Launcher and then play Curse of the Ancient Mariner. Will I be able to fufill the Combo requirement of Mariner, which requires 2 high attacks preceeding it?

once an attack resolves all its values go back to printed. The game "forgets" any modifiers.

Only thing i am unsure of is wether a multiple after resolving becomes a 0m0 attack or remains whatever it copied. But to answer your question, if you play ichi no tachi, then change its zone with bitter rivals or Hata, Once it resolves it is mid for combo purposes.

Stamp for Mr. Mazzurco.

Multiple copies return to whatever stats they were generated at.

Note that continuous modifications will continue to apply though - for example, Melancholic Mercurius's multiple copies will retain the +2 speed from the original attack.

Do they retain the +2 speed after melancholic mercurius leaves the card pool?

No. The static ability is printed on MM, when it leaves the card pool the effect ends.