a) is because as I understand it it's /very/ hard to legally refuse to readmit a US citizen. You can seize all their stuff at the border but besides arresting them for a crime you can't really keep them from coming home.
Yes, you cannot be refused entry as a US citizen even without a passport. You will be greatly inconvenienced, but not refused. You may be held in a detention center or holding facility until such time as your identity can be verified.
In college, friends of mine frequently went out partying in Tijuana and occasionally lost their passports. They were usually held for a few hours at the border crossing and given a hard time and a stern lecture about responsible behaviour before being readmitted to the US.
Note the: as in a person that can prove that they're readily a US citizen. If you don't have a passport, you probably can't readily prove you are a citizen.
The trick is to get the airline to be your police and refuse them boarding. Then your only choice is to get to a Port of Entry via Canada, Mexico, private jet or chartered boat.
Not that it’s the same thing, but being an island (so no walking traffic) and having some screening in place means that the UK simply doesn’t have certain diseases (like rabies) that are at least somewhat common in Europe.
Honestly though, the UK is one of, if not THE, staunchest ally of the US and banning its citizens would cause some minor political issues.
Having travelled back to the UK from a Schengen country in the past few days, I can also confirm that there wasn’t so much as a poster in the airport, let alone any actual screening.
I don't think I've ever had anything that even counts as token screening flying into the UK from Europe - passport check is automated and I've never even seen someone at customs for many years.
https://www.theguardian.com/business/2020/mar/10/eu-airlines...
PS- Good luck!
"Section 212(f) can certainly apply to applicants for immigrant visas. Whether it can apply to an LPR who is returning and not seeking admission is questionable. This is because under section 101(a)(13)(C), an LPR being covered by a Presidential Proclamation under section 212(f) would not cause the LPR to be considered to be seeking admission."
http://myattorneyusa.com/scope-and-history-of-section-212f-p...
"An alien returning to the United States who has been granted lawful permanent resident status cannot be regarded as seeking an admission and may not be charged with inadmissibility under section 212(a) of the Immigration and Nationality Act, 8 U.S.C. § 1182(a) (2012), if he or she does not fall within any of the exceptions in section 101(a)(13)(C) of the Act, 8 U.S.C. § 1101(a)(13)(C) (2012)."