Its hard isn't it ... my MIL has 3 sons; I am married to one, another b@ggered off to Australia and the 3rd just b@ggered off ... She has 4x daily care visits a day, but otherwise me and hubby do it all ... lifeline calls, shopping, laundry, paperwork, liaising with other agencies, medical services, care agencies etc ... just today I have received a letter to renew her council tax relief and last week it was accompanying her with a visit from speech therapist.
If you haven't already applied, get on with an attendance allowance application, to atleast cover your costs for visiting/supporting her, although she must be told this money is for you, as it goes into her account, so she will need to pay you.
Sounds like you need to get Adult Care out to assess her and see what help she qualifies for. Is she able to pay or under the threshold for adult care to help with costs. Sadly, her abilities will only decline, so if you can get some help in place through a care agency now, even if only 1 or 2 days a week, it will enable you to extend this if/when she needs additional care in future. She might not like it, as my MIL is very resistant to any changes, but your hubby needs to get over the "guilt trip" she is playing on him and man up.
I told my hubby some time ago that I would do anything for his Mother, but he must remember I married him in sickness and health, not her, and he soon realised his loyalty to me, outweighed his loyalty to her. Sometimes you need to stand up to hubby as well, and if you also need to remind the MIL that if it wasn't for your and her son's support, she would no longer be able to stay independently at home, then that also need to be said to her face, and repeated, regardless of how small she may make you feel.
What would happen if your hubby stayed away for a day ... would it threaten her life, safety ....? Or just her self-esteem knowing you and he are not at her beck and call all the time ... Tell him to try it ... if she calls you or presses her pendant, ring her back, ask whats wrong, tell her you have a life and need to live it ... it might upset her but she needs to learn to appreciate you again for what you are giving her.
We went away for a few days last month, for the first time in 3 years, partly due to covid, but also as we know MIL is reliant on knowing she can call us and didn't want to not be here. Although it was at the back of our minds throughout, she knew we were not 15 minutes away, and didn't call us at all. We told her the only other option if she was in trouble while we were away, was to ring the lifeline alarm and she would likely end up in hospital, then have to be assessed to see if she could manage at home again ... very harsh but true if there are no family to run when they call ...