@itonewbieThank you so much for answering my question, my practical issue is answered. My theoretical questions are as follows:

[1] You said "The IRS' MeF Schema allows one F.8843 filed electronically with 1040NR". I believe it allows ANY PDF file to be attached, is this right?

[2] The IRS instruction on the form said:

If you are filing a 2020 Form 1040-NR, attach Form 8843 to it. Mail your tax return by the due date (including extensions) to the address shown in your tax return instructions.If you don’t have to file a 2020 tax return, mail Form 8843 to the Department of the Treasury, Internal Revenue Service Center, Austin, TX 73301-0215 by the due date (including extensions) for filing Form 1040-NR

It does not say it can be E-filed, so it is not correct to the letter of law to E-file it? I do agree the risk of compliance is low not filing the form, and attaching the form to E-file, even not supported, make it lower. Since the IRS says attaching the form with the return, it is part of the return, and the PTO should include it I would argue, but it is not a major issue.

Since now I get your attention, I would like to bring up a few other questions I am struggling to get an answer:

[1] The IRS had similar "mail it" statement for Dual-Status return. Suppose it is a simpler case from nonresident to resident, there is nothing different in terms of information being filed if I file it electronically (file the form 1040 as Dual-Status *return* and attach 1040-NR or another form of document as Dual-Status *statement*) or by mail. Is there any difference in terms of transcript and other things after the return being processed? Am I wrong to file it electronically?

I would like to follow the letter of law, and I did mail it, but I get a letter from the IRS that "you didn't file a Form 1040 tax return". I know that “As of January 29, 2021, we had 6.7 million individual tax returns in the processing pipeline” from the IRS update, should I add to that backlog? I would do it if there is a benefit, but I see none except following the letter of law.

[2] Dual-Status individual qualify for recovery rebate credit? I checked everywhere, there is no mention if the dual-status is considered as resident or a nonresident for the purpose.

[3] My other question on if the income (capital gain) is taxable for nonresidents working for US company outside of the US, it is probably against the labor law, but I do not think it affects the tax filing. The original question is in this link.

I would appreciate if you or anyone else can shed some light on these issues, even there are no definite answers. Thanks a lot!