I’m afraid their duty to make “reasonable adjustments” doesn’t mean having to agree to any solution you propose, such as working only one day a week indefinitely. I don’t think many employers would agree to that, and I don’t think the law would insist it’s “reasonable”. Where would you draw the line? If someone insisted they could only manage an hour, would the employer have to accede to that? I don’t think so.
I don’t mean to sound harsh, but reality sometimes is harsh, and discrimination has to be more than simply “not getting what you want”. As the policy presumably applies to everyone, and there’s no suggestion it has been applied more harshly in your case, than to anyone else, I doubt it’s discriminatory. If you could find an example of someone in similar circumstances, who HAD been allowed to work one day a week indefinitely, then I’d agree with you.
On the positive side, your employers clearly are trying to accommodate you, by asking how best they can help, by offering one-day working at all - even if they can’t authorise an indefinite period. In other words, they are offering you a “phased return”, which is pretty normal in these circumstances.
I do appreciate, though, that you can’t commit to a firm date at the moment.
I think you need to check out your pension arrangements more thoroughly. Your mention of “the trust” suggests that you may work for the NHS. In which case you presumably have an occupational pension scheme? If you are found genuinely unfit for work - which at the moment, seems probable - there shouldn’t be an issue with getting your pension authorised.
You already have evidence from Occupational Health that it’s dubious whether you can ever return to work. I don’t know what sort of evidence the pensions people will want, but if you’ve already got professional opinion veering towards the conclusion you “might not” ever be able to return, it certainly looks as if you’re along the right-lines, evidence wise. Dispute may arise if they suggest you might be capable of a different job - just not this one. Many pensions (I think the NHS is one of them) will only pay out if it’s accepted you could not reasonably do ANY job - not just your present one. Proving you couldn’t do any is obviously harder than proving you couldn’t do the one you have just now.
Good luck, whatever happens!