Is it possible to elect portability of my husbands unused exclusion amount (i.e., form 706)?
It is possible but the election really has nothing to do with having filed 1041s and TurboTax does not support Form 706.
You can refer to the links below, along with Rev. Proc. 2022-32, but you will most likely need professional guidance from a tax professional or legal tax counsel.
Rev Proc. 2022-32: https://www.irs.gov/pub/irs-drop/rp-22-32.pdf
See https://www.irs.gov/businesses/small-businesses-self-employed/frequently-asked-questions-on-estate-t...
See also https://www.thetaxadviser.com/issues/2022/nov/updated-simplified-method-for-estate-portability-elect...
