Co-Trustee Responsibilities While Living

Hi all - my grandmother (Illinois) finally got her revocable living trust created at 87 years old. I am the executor of her estate, a co-trustee, and one of three beneficiaries. My portion is a bit larger because the plan is for her to move in with me eventually, we are very close. I also step in and travel to her to take care of her when she is ill or injured.

I have POA for property already, because she trusts me enough to do it that way instead of waiting until she's deemed incapacitated. This is very helpful because at the moment, she's of sound mind, but she just has a little trouble remembering to pay her bills once in a while. This month I set up all of the online logins for all of her bills, set up recurring autopayments, etc. I set up her medicare supplements, I take care of hospital billing from a recent procedure and the follow up care, and I work with her investment broker to ensure the minimum withdrawals are authorized and that her life insurance is paid each year. (She forgot to do that for two years).

I'm not the type of person who will swindle her account for myself or anything, however my mother (her only child) is extremely contentious with the whole family, and is somehow under the impression that she deserves to inherit everything because she is the sole heir. I am anticipating that once my grandmother passes, my mother is going to challenge everything to whatever extent she can. My brother and I are no-contact with my mother, but my grandmother does try to maintain a relationship with her.

My grandmother is often generous with gifting my brother and me with help for things like our kids' education and contributing to the down payments we made on our homes. She bought my mother a home outright but kept it in her name, the deed will go to my mom upon her death. She also reimburses me for plane tickets or rental cars when I travel to/with her, little things like that.

My grandmother wants to live at her home as long as possible, but she and I are planning on upfitting an accessory dwelling unit at my house for her potential use eventually. It seems prudent to do those renovations this year because they will take a few months and I don't want to do that right after she has a stroke or something. My fear is that if she doesn't end up living in it, (or even if she does) my mother is going to try and say that I was using grandmother's money for my own benefit and home improvements while I was a co-trustee.

What should I be doing now as I basically sign checks for her to pay her bills, do this renovation to the granny-shack, or any other financial transactions while she is living? Do I need to keep a notebook of what each expense is for? Is that enough? Do I need my grandmother to put in writing that the money we are spending on the renovation is with her permission? I hate that I have to prepare for the fight, but my mother will 100% bring the fight, and I want there to be no question in the future that what I'm doing now is on the up-and-up. The trust has an in terrorem clause, but if my mother truly believes I was acting in bad faith, I need to be able to prove later that I am not, if it comes down to it.