I went through something similar a few years back Marty.
My advice would be to get that 2 weeks sick note extended to 4 weeks ASAP. This will give you a bit more breathing room, time wise, to assess exactly what you're going to do. Your doctor should sign this off no problem if you are firm with them that nothing has changed.
I'd definitely echo what Baron said regarding documenting everything, and making sure this is all documented in writing, should the need come for anything to be looked at further. I'd request evidence/examples of what these other 'managers' have said, and ask for exactly what the next steps are to be once this 4 week performance plan is finished (I'd also ask exactly what this performance plan is?) I'd purely request these to push back on them, but I wouldn't hold my breath for anything.
I think you only being there for such a short space of time means the bottom line for me though is that there's no coming back from this (definitely from my experience of hearing people on performance plans, it's basically a push to get somebody out) so you need to try and clear your head from their noise, and focus on yourself and what's to come. You've said that you you don't want to go back anyway, and I'd imagine unless you are very comfortable financially, a job is needed in the short/mid term, so over the next 4 weeks I'd make sure you're structuring your days to get yourself back on track.
Set aside time for yourself each day, exercise/gym if possible, seeing family/friends if possible, whilst also making time each day to focus on your next career steps. Plenty of Marketing jobs out there at the minute, get your CV dusted off, get yourself on job sites/LinkedIn etc and get some career positivity and pride back by securing a few interviews. You can then hand your notice in on your terms, rather than being dismissed.
Good luck mate!