-chau is a colloquial form of -te shimau (-te shimau -> -chimau -> -chau)
-te shimau is used primarily to state completion in the near future.
(literally, shimau means to "get something finished")
Consequentially, it gives an impression that the stated action is unavoidable.
From this, -te shimau is used to express speaker's resolution (when stating the speaker's
intentional action) or regret (when the result is unintentional)
-kanaa? is used to add some uncertainty to the statement,
to soften the bluntness or to seek approval of others.
Very roughly, your examples translate like this:
Sorede kaeru = I'll go home after that
Sore de kaecchau = I'll be gone after that
sore de kaecchau kanaa? = (I think) I'll be gone after that. (Is it okay with you?)
I suppose you're wondering Sorede kaeru vs Sorede kaecchau, or kaeru vs kaecchau.
Kaecchau is a casual form. And these pairs don't differ in meaning very much: either way they're suggesting that somebody will/end up going back to somewhere or home. Sorede is "And then,", suggesting the next step.
Sore de kaecchau kanaa?
This sounds to me like you're wondering that somebody will/may go back to somewhere or home. But I'm not sure, and that depends on your context.
We have an old manga called "Taiho Shichauzo", which says "I'm going to arrest you". Note that -chau varies with its use.