I posted a similar question on the 07/10/22 - Studio One 6 - Plugin missing.
It was my first post so maybe it did not make sense as no replies yet.
I have a Mac Mini M1 and opted for 16Gb of memory + 1 Tb SSD for ‘production’ work and use an old Toshiba Satellite i5 with 8G ram running Windows 10 for gigs, I am not a Windows fan for for music production and used to run Linux with Ardour prior to trying out Studio One in 2014, cannot wait for the laptop to give up and die!
For the likes of us life is not simple, to make matters worse we all have different gear and approach solutions differently.
Last week I also updated to Studio One 6 and installed it alongside Studio One 5 on my Mac M1 and fortunately left the install on my gigging Windows 10 laptop for later, good news and bad news, I can confirm that if you use the same install file as I did for the Mac M1 it does work with Studio One 5.5 and still does, so like myself that would at least give you something you can work with whilst we find a solution to our problem with Studio 6.
My download was from here, bear in mind this may have been updated since the 15/05/2022:
https://www.jamorigin.com/download/
https://www.jamorigin.com/downloads/221/MIDI-Guitar-2-for-MacOS-10.15-and-later.dmg
I have just looked at Get Info and nothing for Rosetta on Midi Guitar 2 whereas the Midi Guitar Mini app I installed does have the option, but not ticked.
This all applies to the Mac M1.
In the Studio One 5 browser MIDI Guitar 2 is listed in VST2 and MIDI Guitar Mini in AudioUnit.
In the Studio One 6 browser MIDI Guitar 2 is not listed and MIDI Guitar Mini is still in AudioUnit.
My logic now says for me to try MIDI Guitar 2 as an AudioUnit plugin even though you stated that this is what you have done, at least it will confirm the outcome on a different system.
I have just re-read your post and am not sure whether you have set the tracks up correctly, it sounds different to the way I have set it up, I will check this out later due to time constraints now, I did find a guide somewhere when I first set it up.
Out of interest there were a couple of other plugins not working in Studio One 6, I can list them later if anyone is interested, the good news is that Manda Audio have just released a new version of the MT Power Drum Kit so that one is solved for Studio One 6 on the M1 if someone else has the same problem. I use this as a super click track to be replaced with my acoustic kit live later.
Fishman have also updated TriplePlay so I will also be trying this out as an interim solution until the MG2 problem is resolved, just means the inconvenience of having to take an extra guitar (Fender/Fishman Stratocaster) out, MG2 had solved this as I can/could use any guitar as a source, even my old Line 6 Variax 500 that works seamlessly with the Helix Rack for my guitar sounds.
The other option is to use Studio One 5 for gigs to keep the use of MG2 for live work, will have to see how this works out in practice.
I will update this a.s.a.p., hope it has helped.