It sounds sharp but many do. Specifically it depends on whether it is worth it, for example do they have a substantial name and rep that would open doors, are they pulling in contacts etc. it also depends on your music genre as arrangement and production can have just as much to do with commercial success with say club or pop music, while less so in the country or folk scene.
In many scenes they would be more likely to get 5 -25%. Commonly they would be negotiated on a per song basis, but it could just as easily be done on a group of tracks...
The point is it comes down to negotiation. One point to make is just because they want 70/30 doesn't mean they get 70/30. It is a negotiation. If you are prepared to say no and walk away then their offer might improve... But even if it doesn't you do not need to accept it.
Have you paid them too? Are they contributing to the financial side of the EP at all?
Regarding your songs... If you have proof of what you wrote or didn't then what you have proof of is all you can walk away with and not have. Deal in place. Incidentally you could say to them regarding the tracks they wrote and produced that you contributed to the performance and if there is even a small ad lib you could likewise tie them in knots as like you the other way around, they didn't get you to sign a contract so you could get very sticky with them.
Regarding your adlib and all the rest, it depends on a need to preserve a working relationship or not. For example if you wrote it you are the writer... But proving it may be another matter. To that end if you walk away simply rewrite the adlib, completely. Same goes for the song the wrote a word for... Replace the word and go back to your original or write a new word and ad long as you can prove you wrote what you wrote and didn't use their work at all... It is ok. Pretty well.
Don't take my word, you need to see an entertainments lawyer ASAP. I am not a lawyer.