Is the Fadeout value the only way to modify the release section of an envelope? No way to change between linear/exponential etc?
Furthermore, the curve used for the fadeout sounds like it’s shaped a bit like -¬, i.e. at the very end it dies out too quickly. Is there any way I can achieve a slow, gradual, exponentially decreasing release phase? Something like this:
XXXXXX
X
XX
XXX
XXXX
XXXXX
Or just a linear one would do too.
Nope. You simply need to turn on sustain for that instrument, and then create an appropriate volume envelope.
http://tutorials.renoise.com/wiki/Instrument_Editor
The current fadeout behaviour is linear, as far as I know.
Edit: Sorry, I typed exponential there when I meant to say linear. What I was quickly testing actually revealed something that looked exponential, but it was unrelated.
Here’s an example of the output from an instrument with fadeout:
OK. It seems I misunderstood. I had sustain on from the beginning with a nice release curve after it but the sample finished playing before the curve had finished due to the Fadeout setting. Which made me believe that Fadeout controlled the release and not the curve.
Just realised that setting Fadeout to 0 and drawing my release curve as I want it does the trick!
Aaahhhh, ok 
Yes, I’ve actually ran into this myself a few times. It’s easy to overlook things like this.
Be careful of doing that if you use loops. From what I remember it doesn’t release and your best settings values of Fadeout to be sure you don’t have the sample playing but at silence for ages at the end of the envelope.
Hmm, I don’t use a loop but it’s a very long sample, and I noticed that the sample keeps playing after the envelope has reached zero (the line in the Sample Editor is still moving). Increasing the Fadeout to 1 doesn’t fix it however.
Keep on going up with the Fadeout until you see it. Scale seems a bit weird so needs a bit of experimentation.