No!
No, no, no.
Just no.
In Bitcoin, there's no such thing as a "sending address". DO NOT try to guess at a return address from the inputs of a transaction. Doing so WILL eventually result in lost bitcoins and angry users. If you want to send bitcoins to someone, ASK THEM TO PROVIDE a receiving address, then use the address they give you.
There are just SO MANY ways that guessing a user's address from inputs will result in you using an incorrect address.
You're probably right, some users will ignore even the simplest of instructions. The return address only works when using your own wallet. This badly complicates my idea.