You can start using Ike after Part 3 and Sothe also appears during Part 3. But Sothe really isn't a good character.
Part 4 Chapter 3.
I would have to say III
You get her automatically if she survives the chapter she betrays Begnion.
You can get that staff by standing on a certain square in chapter 3-13
on Part 3, the chapter with the river crossing with the Greil Mercenaries, get Ike to fight the Black Knight. Don't let anybody die and in the Endgame, you can get Sephiran. Ike needs at least 27 speed. My way is to attack the Black Knight, then rescue him with Haar.
Part 4, Chapter 3, Distortions. Baselard is at 12E and 10N.
You can't. There are no cheats in Fire Emblem Radiant Dawn.
You meet him at the end of Part 2, after Elincia's Gambit, in a cut scene
No, Radiant Dawn is only for the Wii.
No she died in Fire Emblem Path of Radience
in a store...
Yes, there are.