Aha!! So you thought I was already abandoning, huh!? You couldn't be more wrong!
Although this has been quite a busy week-end, coming back home really late and too sleepy to type a word here, I still have dedicated few hours every day to the synth (before going out!).
I have implemented the variable value ranges in the envelopes, so that I can select whether I want the envelope to start in negative or positive values (from a range of carefully selected meaningful values), and fixed a few quirks here and there.
The pitch envelope is already in place too -- producing some interesting strange weird effects.
There's still a little aesthetic issue with Renoise, but I am pretty confident it's got a very easy solution. In a bad case I guess I can just "ignore" the values which are shown on screen and rely only on the position of the sliders and what my ears hear.
The only thing I'm unsure about is about updating the envelopes for each sample or for each call to the getBuffer function. The first one will be finer but more computationally intensive. Not that it is that intensive. I guess the solution is to try it and see if it sounds better...