The §645 election (Form 8855) is due by the due date (including extensions) of the first Form 1041 for the related estate/fiscal year—not by the decedent’s 1040 due date. For a fiscal year ending 9/30/2025, the first 1041 is due 1/15/2026 (15th day of the 4th month after FYE), so 8855 is due the same day (or later if you extend the 1041)

It is not necessary (and doesn’t help “matching”) to file the 1040 and 1041 on the same calendar day. 1099 matching is TIN-based, and your 1099-B is under the EIN. File the 1040 by 10/15/2025 as planned. Then mail/file Form 8855 and file the 1041 any time on or before 1/15/2026 (or on extension). This is the cleaner path and ensures the §645 election is properly on file before (or contemporaneous with) the first 1041.

If an executor is appointed after a trustee makes the election, be aware a revised 8855 may be required within 90 days if the executor agrees; otherwise, the election can terminate.

"2024 1099-B is in the name of the trust w/EIN; step up in basis is included on 1099-B post DOD . . . I have entered 1099-B totals on both 1040 + 1041 with separate line to adjust for nominee distributions; separate line references adjustments showing SSN on 1041 as well as EIN on 1040 so each return reports the correct amount."

This makes no sense.  If the post-death 1099-B already reports using the EIN, why complicate the 1040 with irrelevant transactions?  Why put sales reported to SSN on the 1041, unless there were some transactions before the new account was established?

And no, there is no short little man with a green eyeshade scrutinizing these returns as they arrive at IRS.

The 1099-B contains all 2024 transactions - pre DOD and after. Broker did not open an new account upon DOD.

So, pre-death sales are reported on a 1099-B with the trust EIN?  Then the only nominee distributions would be on the 1041, if you want to bother with that adjustment.  (IRS has threatened to match 1099s  with trust returns, but reports of it actually being done are few and far between, especially when they know the grantor is recently deceased.)  The pre-death transactions would go on Form 8949 as "not reported on Form 1099-B," because they were not reported to the taxpayer.  

And this is why most brokers will open a new account when a trust becomes irrevocable. Usually there is a change in the trustee, didn't they want to know if the new one was a US citizen?   Looks like  sloppiness in compliance with rules for financial institutions.  

Thanks @strongsilence and @BobKamman for replies - all info provided was helpful!

You are welcome.