From what i can comprehend from your post, you want to know if you can plug your mp3player + an rca input into your amp. Amps have RCA inputs on them, simply plug the mp3player into these, then u dont need AUX/RCA inputs. If u did need another input (for a discman or something) you can use a RCA switch box to select the source you wish to hear.
and as to the second part, if you plug any thing straight into the anp, the tape deck will not control volume, u'll either have to control volume using the soundcard (software control) or make an external volume control (read the thread in Hardware Discussions relating to volume control) or use a pre-amp.
hope that answers your question
My plans and progress on my install http://www.mp3car.com/usersites/hornet