Also, if a jutsu resurrected Yahiko to where Yahiko regained his soul

[Kadmos1]

Why do you think Nagato decided to use the corpse of his best friend, Yahiko, to be the "main" Pain corpse? I think Yahiko would consider it horrendous that his body would be used to act as the leader for a terrorist organization that seeks global domination.

Also, if a jutsu resurrected Yahiko to where Yahiko regained his soul but still had Nagato's Rinnegan (Since the Tendo corpse was a puppet being controlled by Nagato and the 2 bodies shared "minds", I think Yahiko could keep those memories) and sided with the Konoha shinobi, do you think he would as near of a threat against the 3 remaining Uchiha as Naruto is?

I think the fact 1 good shinobi with the Rinnegan should be something that Obito should fear because he'd be going up against someone who figured that Obito isn't Madara.

I think that though Yahiko would be limited to gravity attacks, he could really do a number on Obito or Sasuke.

 

[CurseSealofEarth]

Out of love for his friend. He killed himself (sort've) so he probably wanted to keep him.

 

[The Jess Dogs]

i think yahiko was used as nagato believed he was the face of the akatsuki and wanted him to lead them