In SoA, I told Keldorn he can leave and stay with his family.

Will I be able to summon him in ToB?

Yup. The only ones you can't summon are NPCs that you left dead or imprisoned.

and yoshi :(

who, most likely, is dead :D

You can safely lose the "most likely" ;)

Why hasn't anyone made a mod where you can save Yoshi? I'd play a mod where after you clense his heart at the temple he gets ressurected to help you fight Jon. Of course he would be much lower in level than the rest of the party, but maybe the heart thing could give Yoshi a lot more exp than it gives to the rest of the party. I know some quests reward a certain amount of exp to each person of the party instead of a set amount divided between everyone, so the quest could reward everyone a set ammount and then yoshi a little more.

There is one -- Tortured Souls.
With that you can choose to take Yoshimo (and a new NPC, actually with voice set) to the very end of ToB.