Your T may well be hanging onto her job as she doesn't want to retire. I guess it would make it simpler for you if she decided enough was enough but, as you say, all you can do is make the decision about what is right for you given the situation now. It does sound very uncertain especially as she wasn't back when she hoped to be. Given her age and the uncertainty, it comes down to whether you can accept the inconsistency of the work with her and find enough value in it regardless, or make the decision to work towards an ending.

I'm sorry, it's horrible when a T's health dictates a decision like this.