Doubts about Sine wave/ Audio routing

Hi houze and welcome

I can have a go at the first question.

Yes, you can feed audio into the AE Modular, although you may need to use a 2SIGNALAMP to boost the signal - I had to do that to feed some audio into my rack. Depending upon how much feeding in and out you want to do, you may also need the 4I/O to give you some extra ways in and out.

It definitely works and is you can manipulate your sound quite nicely. I recently bought some contact mics as well and I've been having some fun with those - I certainly needed the 2SIGNALAMP for that.

Regarding to the sound amplification, if you don't mind your hands to get a bit "dirty", I'd suggest you for the sake of experimentation to try pre-soldered 358 op-amps, they're super cheap and very easy to implement in AE modular, especially if you're experimenting with something.
I've bought a few, made a dedicated module. When the amplification if set to max you're getting some distortion, but besides that, if you can control it and lower the volume a bit - it's just great and amplifies the signal pretty well.
Here's where i've got mine, but I believe there's a plenty of other and similar places for that: www.ebay.com/itm/142659909601?ul_noapp=true

The future sounds very exciting

Additive! OK, now you're talkin'! Would it be something elaborate along the lines of Mark Verbos'/Buchla's harmonic oscillator (separate level controls per harmonic + per-harmonic outputs) or a bit more like the new Xaoc Odessa?