About Sonya I agree that IADK and D4 are her best tools to keep Cage out, and her projectile. But from a practical point of view I think that once Cage gets in he just has to be a little more unreadable than against other characters, especially when he decides to take a risk and go neutral.
With Ermac the way I play the matchup is by not spamming TKP from anywhere other than max range, but rather to make the opponent respect it and get in for a throw mix-up. Still, I think that Ermac can spam TKPs, I just don't do it that much. He will get a nice chunk of meter for breaker while Cage tries to get in. And remember, Ermac's zoning is quite tricky because it's not just projectiles, but pushes, F4s, throws (more than other chars, because it sends you fullscreen), then balls and well-timed aa lifts.
Also I should repeat that Ermac's raw, meterless damage, really helps him a lot.
With Jax I believe he can play footsies, but that's just it. Not sure, but you tell me if Cage doesn't have the upperhand in that aspect. Jax gets fucked from JC's low hitbox.
Also, I forgot to say how happy you made me to put KL losing to JC.
Cheers