Hi Conner,
This is great stuff. 
I am not a coder, but I’d be happy to help out where I can.
Volume
The minimum value is 0, the maximum is 127.
There’s a good reference to the MIDI spec here
http://www.borg.com/~jglatt/tech/midispec.htm
that explains most of the parameters.
This is the time base for the MIDI file. It’s usually expressed as “clicks per quarter note”. Renoise uses “Ticks” as the time basis, and the time is a factor of BPM, Lines Per Beat and the Speed setting. There’s an ongoing discussion about it here:
https://forum.renoise.com/t/a-question-of-speed/12837
You really need a Renoise dev to define this for you, unless you can comprehend the maths. I can’t. 
Instruments
I don’t know if there is any way to convert the Renoise concept of instruments to the MIDI spec, which is based on General MIDI instruments. They don’t have any simple correlation to the Renoise sampler or to VSTi’s, though you might be able to extract the patch information from the VSTi data if you really wanted to. I think most people would be happy with just the note information.
I would extract the Track information from within the tags rather than worrying about assigning instruments.
BTW, what made you choose MF2T/T2MF over MIDI XML?
Cheers,
Malcolm.