Maybe it's the real estate agent instincts kicking in and they're not thinking of the whole situation.
They might be counting on Ultrarunner to not only go above and beyond as usual, and not only for free, but against his own financial interest and against his personal connection to his Mom's home so the home can be sold for top dollar to whomever instead of it staying in the family.
From a fiduciary viewpoint, a trustee cannot engage in speculation with trust assets or make expenditures not authorized by the trust terms. If UR makes an unauthorized expenditure to improve the mom's home, the real estate market could drop or some contractor could really foul up the improvements leading to a loss of trust assets for which UR would be responsible from a liability viewpoint.
If the trust says it ends at death of the surviving spouse, it's time for distribution. Period.
The same issues apply to an Executor. Follow the terms of the will. Period.
The only safe option for a trustee/executor is to exactly follow the terms of the trust and the will, and never think for a second he can do things with fiduciary assets that aren't expressly authorized by the written terms just because the beneficiaries want it done like they want it done. That's not how wills and trusts operate.
And even if by some wild stretch of imagination some California court authorized the Executor to improve the property, the probate court can only do this with estate funds because the trust funds are by design not part of the probate estate.
Even if the court authorized UR to improve the property, UR could decline to serve as Executor or resign leaving the siblings to deal with the improvements and/or handle the estate administration.
If the property ends up passing to the 3 siblings as co-owners, then there could be a deadlock where the 2 would have a lot of problems trying to improve or sell without UR's approval. If the deadlock persists, their recourse is to file a partition action to sell the property.
An independent lawyer might be the one to get the message to the siblings that UR can't be expected to work against his own interests so they might come to understand that they ought to just take the offer and be grateful for it.