I’m really sorry, but I can’t advise you of anything you can do.
If you raised your objections, but there was a democratic vote, and you didn’t prevail, I don’t see that it’s in any way your fault.
In your capacity as a senior manager, you recognize that despite the obvious benefits to you personally, the existing arrangements were so generous as to be unsustainable.
Are you able to start putting aside any money NOW, so that IF you were sick beyond the period they’ll still pay for, you’d have more of a buffer? I know it’s difficult for anyone to build up a nest egg, these days, with salaries frozen for years, for most people, or rising at less than inflation, and groceries and utilities going up all the time. But if you could find even a little to put aside whilst you are on full pay, it might give you some safety net if you’re off sick again in future.
Also, lenders won’t foreclose immediately, if you’re having trouble paying. If it does happen that you’re off sick, and, under the new terms, reach the point your salary stops, you will need to contact the lender immediately, and explain what’s happened. You will need to make clear (and possibly provide proof) you haven’t lost your job - this is a good employer, and you’re merely off sick; there’s every prospect you’ll return to full pay in due course.
The lender may be able to offer some compromise options, including a payment “holiday”, or a temporary switch to interest only (if it wasn’t already), so your monthly payments would be smaller. In most cases, they don’t actually want to repossess (housing market’s not great at the moment, so chances are they’d have to sell at a loss, and wouldn’t recoup the loan…)
So a borrower who is proactive and shows they are taking practical steps to avoid defaulting, during a temporary crisis, is likely to receive some support and cooperation. Obviously, you don’t need to tell them now, before it happens, but IF it does, don’t make the mistake many do, of trying to hide it from them. Approach them straight away, and explain, and see if they will cut you a deal.
Also check in detail which benefits in addition to SSP you may be entitled to, if your income falls below a certain level. It used to be possible to get assistance with mortgage repayments, in certain circumstances - but I don’t know if that’s still true - or if it ever was in Scotland. But I would strongly urge you to investigate this before your next relapse, so you know exactly what you could claim, and when and how. Don’t wait 'til you are already on the sick, and in a panic, as obviously that’s not the best position to research what might be available. You could go to the CAB now, and ask them about it - explaining that whilst the situation hasn’t arisen yet, it’s very likely in future, and you need to know what to do.
Ethically, I don’t think you’ve done anything wrong at all. As a manager, you have to look at the wider picture - including, ultimately, the survival of the charity. You can’t draft policies to benefit only you, and others like you, if it would be financial suicide for the organisation. And let’s face it, that wouldn’t benefit you in the long run, anyway. What’s the point of getting indefinite 100% sick pay, if the result is that when you’re better, there isn’t an organisation to go back to? It’s clear from what you’ve said that the most likely result is none of you will have jobs, if the present policy goes on unchecked. You can’t let that happen. It’s better that employees - including yourself - have less generous benefits, than that they don’t have jobs at all.
I’m wondering, though, if there’s any room for compromise? I suppose it depends on the turnover of staff. What would the books look like if, say, you phased it in, only for new employees at first? Would that still be unsustainable, because most staff are long-stayers? Or made it more gradual, so the changes don’t come into force literally overnight, but there is a transition period, where people’s benefit expectations gradually reduce, until the new terms have been fully implemented, but they don’t come in all at once?
I realise that saying, for example, that there would be no changes until next year, or that the initial limit would be six weeks instead of four, or tinkering with it in other ways, wouldn’t fix your problem. Whether it’s now, or in five years time, you’re eventually going to run up against it that you’re off sick more than is covered by the prevailing terms. But having some kind of phased implementation would give employees (not just you) some time to think about how they could protect against the shortfall. Employees who haven’t already been diagnosed with anything could take out income protection insurance, for example - although I realise that would not be open to you. I think your main option is to start saving - as much as possible, as soon as possible, and to look into what routes are open to you if your pay does ever stop.