The idea is solid, the implementation may leave something too be desired. A hesitation I have is that Lemmy is new and unstable. It can be tricky getting things to work right for the average person, I can't imagine it's an easy time for anyone with higher accessibility requirements.
Unable to suitably categorise Posts (sub-group/topic type/region)
I think the only thing on Lemmy that helps with this is creating different communities on an instance. There is currently no tagging of posts or anything like that.
Difficulty searching/filtering
I'm not sure the search on lemmy is all that great? And with no tagging, filtering is pretty non-existent.
Posts ‘drop off’ the bottom (due to inability to filter)
Lemmy lets you go through each page so you shouldn't have a problem with not being able to go back far enough. It also allows you to sort by new comments so it's easy to find the things people are commenting on.
Continual hacking risk
This is a risk for any platform. I guess one benefit is you have your own backups, so if something happens you can restore from a backup. But I'd think Lemmy is much more likely to get hacked than a facebook group (with admins using 2FA). It wasn't that long ago that a security issue in the Lemmy software caused the largest Lemmy instance to have an admin account taken over by a malicious user.
Platform controlled by off-shore corporate interests
This is a benefit of Lemmy. You control it, you host it, you own it. You can choose which other servers you federate with (if any).
Also, it seems like this might be something that is helpful to get a wider range of opinions on. The community on !newzealand@lemmy.nz is significantly larger than this !support community, perhaps you could cross post it there as well? (there is a little icon under the post for cross posting if you're accessing Lemmy from a browser).
Also r/blind set up a Lemmy server at https://rblind.com and they may have some suggestions on whether it's suitable.
One thing that may help that people often don't know, when you post a picture in Lemmy (or anywhere using markdown), you can put alt text so blind users can have their screen reader tell them what's in the image. You put the alt text in the [], like so:
![alt text](https://image.url)