Yes
There is no giratina in almia. The only legendaries are the gem gaurdians, events, and the regies.
no you need to have the three regies in your party to make regigigas appear
you must trade them
Regice-Almia Castle, near Magmisguis (need magmortar) Regirock-Cave on island off coast of where Blastiose is in Vien Forest ( need garchomp) Registeel- Chroma Ruins near end(need Gallade Regigigas- Unknown ( would like to know)
you cant. you have to trade them from Pokemon emraled.
you need to have the national pokedex
Trade or wait for an event.
Yes you do.
Unfortunately, you must migrate the regies from ruby or sapphire. You cannot catch them in pearl/diamond.
you can use cheats or have a regigigas from the 11th movie
You keep fighting them holding the friend bow
Get a higher rank and face them again