1. The instructions say to mail the entire form.
2. This is a final return since it reflects the final disbursements of the estate.
3. Yes, I would include that detail.
4. I would use the date of death if you choose to file for 2021, and the date of receiving if you choose to file for 2022, to avoid any confusion at the IRS side.
As both @pk and I have mentioned, the doctrine of constructive receipt allows a reasonable argument that you did not receive the funds and property until they were actually transferred to you, even though the law also provides that the value of the property is determined on the date of death. However, if you prefer to file for 2021, I don't think the IRS will find fault. You are not reporting and paying a tax obligation, this is a reporting requirement that is put in place for other reasons.
https://www.irs.gov/forms-pubs/about-form-3520
The constructive receipt of income doctrine has long been a part of the income tax laws. Under this doctrine, a taxpayer will be subject to tax upon an item of income if he has an unrestricted right to determine when such an item of income should be paid. This principle was expressed in a 1930 Supreme Court case, Corliss v. Bowers, 281 U.S. 376 (1930), in a statement by Justice Holmes, that "Income that is subject to a man’s unfettered command and that he is free to enjoy at his own option may be taxed to him as his income, whether he sees fit to enjoy it or not."
The doctrine is embodied in section 1.451-1(a) of the Income Tax Regulations, which states that an item of income (for example compensation for services) is includible in gross income for the taxable year in which it is actually or constructively received. Section 1.451-2(a) of the regulations states that income is constructively received in the year in which, although not actually received, it was made available so that the taxpayer could actually receive it at any time if notice of intention to receive it has been given. However, income is not constructively received if the taxpayer’s control over its receipt is subject to substantial limitations or restrictions.
