"I have basis (from the day the Trad IRA was converted to BDA) "
The traditional IRA becoming an inherited (BDA) IRA does not create any basis. Assuming that your brother had no basis in nondeductible traditional IRA contributions, the entire amount of any distribution from the IRA-BDA is taxable income.
Once the IRA-BDA is split into separate IRA-BDAs for the estate beneficiaries and assigned out of the estate to the corresponding beneficiaries, any distribution will be paid and will be taxable to the beneficiary who obtains the distribution from the IRA-BDA maintained for that beneficiary. No Form 1099-R can be created for the division of the original IRA-BDA into separate IRA-BDAs because such a division is permitted to be done only by nonreportable trustee-tot-trustee transfer, which is neither a distribution nor a rollover. An actual distribution from any of the IRA-BDAs is reportable on Form 1099-R, will be taxable, and cannot be returned to any IRA-BDA.
"I've read other posts within this forum in which some are populating 1041 Line 8 with the FULL value of their IRA-BDA."
What gets reported on line 8 of Form 1041 is whatever amount is actually distributed to the estate from the IRA-BDA. The full value of the IRA-BDA would be reported on line 8 only if the entire IRA-BDA is distributed to the estate. Of course if that happens, that's the end of IRA-BDAs in this case.
IRAs contain deferred income and any distributions (and only distributions) are Income in Respect of a Decent (IRD).
If you haven't already done so, review IRS Pub 559:
https://www.irs.gov/pub/irs-pdf/p559.pdf
