New Job Offer while on STD

I’m guessing they would terminate you but I think it is really hard to prove they couldn’t accommodate you.
If every job in that industry is high stress then maybe you can’t work for any company in that industry so the company could prove they couldn’t accommodate you (my non lawyer opinion).

I don’t know about severance.

If your employment contract has become frustrated, then your employment is deemed to end immediately and you have no right to working notice or severance pay in place of working notice.