Great list.
For the Kontakt instruments you can run them for 15 minutes in Kontakt Player with Demo mode, after that the sound will be off until you reload the plugin. What I did in the old days is to arrange the notes until time out or after, then reload Kontakt and render it within the 15 min limit.
NeonSpider
There are lots of free and open source software (FOSS) for music production. Some excellent. Some terribly broken. Some have huge learning curves but you can do amazing things with them if you get past that. You've mentioned a couple such programs (ZynAddSubFX and Ardour), but I think you should look into the following and see if you like any of them.
Audacity -- sound recording and audio and waveform editing
Hydrogen Drum Machine -- a software drum machine
MuseScore -- great for producing sheet music from MIDI files or you can compose in it directly
Rosegarden -- a software sequencer type of thing. Will require JACK and some softsynth
LMMS -- IMO not as good as Rosegarden but can be used for a similar thing
FluidSynth -- pretty much the "default" softsynth that is recommended be used with JACK but you can use others instead
QJackCtl -- GUI that makes JACK a bit easier to use
QSynth -- GUI frontend to FluidSynth (at least presently) though perhaps not indefinitely (it might use some other softsynth at some point)
Yoshimi -- similar to ZynAddSubFX and in fact built from it I believe
MilkyTracker -- just an excellent music tracker software similar to FastTracker
OpenMPT -- probably one of the best music trackers around
PureData -- very complicated huge learning curve but incredibly powerful music programming environment
SuperCollider -- another complicated huge learning curve but powerful music programming environment
OpenMPT is Windows-only software, though it works fine used under Wine on Linux. The rest can be used on Linux natively.
Audacity, MuseScore, LMMS, MilkyTracker, and PureData all should be able to be used under Windows (and probably Mac as well). Not sure about the others. Obviously JACK is more of a Linux audio thing and a bit non-intuitive at first.
The Windows version of PureData is not entirely compatible with the Linux version and so forth. Meaning, depending what you do, it is very possible to make something in PureData which only works correctly in Windows, which only works correctly in Linux, or which only works correctly in whatever OS port you are using.
There are lots more, but these are probably worth looking at.
LD-W
Thanks for posting!
I'm definitely familiar with alot of the above packages since I've dabbled with composing in a Linux environment before in the past (I did recently hear that yabridge shows very promising results for allowing alot of both VST2 and VST3 plugins to run in both Bitwig and REAPER on Linux, with other DAW's with yabridge being in variable states of support). What I may do for certain things like Audacity, Hydrogen and Musescore is add them into a separate category since they're not exactly fully-fledged DAW's, but still very useful. I'll add OpenMPT to the DAW list shortly. I'm still very 50/50 on LMMS being a recommended choice right now (it felt unstable and clunky with a fair few mainstream VST's and samplers when I last tested it), albeit it's been awhile since I've bothered to really give it a go.
The main intention with the list for now would be as an archive of easy, ready-to-go resources for composing/scoring/drafting that anyone at all levels of skill/technical-competency can just grab and use with minimal fuss, which is why I've left JACK and it's associated packages off this specific list. I think what I'd like to do for that instead though would be a separate list for Linux Composing, so we can also include other things along with JACK-packages such as recommended LV2 plugins and decent/promising wine-staging packages for running VST's like yabridge etc. Plus some straight forward documentation and recommended distro's to let anyone interested dive into trying it out without too much issue.