Like others, I think you can get a better accounting than that, if what you got was what I imagine from your description. And I think a clear accounting should set the mind at ease. Certainly it's missing the supporting documentation, and I suspect you're within your rights to ask for that. But I think you're almost done.
Based on your description, what you are proposed to receive is reasonable...assuming you are willing to accept your cousin getting free rent, when probably you and your brother are technically entitled to a portion of the forgone rent. I'm not a lawyer, so just guessing (I think some of the other commenters ARE lawyers...). From what I think you said, it went like this:
Original estate
115k financial assets
100k house (estimated)
-----
215k
- 2.5k executor fee (not crazy...though 2500 makes me wonder if it's 1% of 250k, and 35k wasn't accounted for?)
- 41k debt payments, funeral expenses (may be reasonable, should have been accounted)
____
171.5k assets to be divided by heirs
Mom gets 171.5/3 = 57,167 or so.
Mom got 20k for something that you found out about later, without clear accounting (should have been accounted)
57,167
-20,000
---------
37,167 left for you and brother to split
It could be bad accounting in good faith. It could be bad accounting that was done after your lawyer's accounting request made them back down from asking for the "waiver." It could be accurate accounting that just doesn't include the supporting documentation. Perhaps it's close but a few details were hard to document and they're sweating. If the 41k "debt" paid mom's siblings for labor, more judgment is required, but if the 41k paid actual vendors outside the family, the accounting could be pretty accurate. I'd just ask for the supporting documentation.
Re rent - Suppose rent for the property should have been 15k at fair market value for the time Cousin stayed at the house after Grandparents died. Mom should have received 5k from that. That would be 2,500 each for you and your brother. I wouldn't find fighting over that amount to be worthwhile, and if I trusted the accounting, I'd take the 18k. I'd be comparing the amount vs the cost of arguing.
I'm a persnickety dude. Asking for docs is in my wheelhouse. But if you got bank statements, and any investment statements, and they support the accounting that was made, you'd be golden unless the amount of the rent is important to you.
Take that walk, like the other poster said. But I'm guessing that this response was the acknowledgement that you "won" against the earlier request to waive your mom's share, which has been implicitly withdrawn. I think you're in the home stretch now.