What do they mean by this in the UK GOV website for Transit Visas? And other questions [closed]

I need to immigrate to Boston Before JUNE 18 (the date at which my immigrant visa expires) . I have a ticket to London where I arrive at 18:30 Local time. The second ticket I'm considering to book leaves the next day at 11:15 am May 23 to Boston from Heathrow Airport. According to the UK gov website, I won't need a transit visa according to some conditions that I know I fulfill the most of, but there are some I'm not sure of, one of which is the one below:

- Have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive

Does it mean that if I get to London at 18:30 On may 22, I can leave before 23:59 On may 23 without needing a transit visa?

Also, while you're at it, the UK GOV website says that to transit without a visa, I need to fulfill: 'you’re travelling to (or on part of a reasonable journey to) Australia, Canada, New Zealand or the USA and have a valid visa for that country'

I have an F4 immigrant visa, does that qualify?

Also, There's the EU travel ban. Does it mean that I can't enter the US? Cause my visa is just an F4 immigrant visa. However, on the Presidential proclamation it says that this classification is excluded:

(v) any alien who is the child, foster child, or ward of a U.S. citizen or lawful permanent resident, or who is a prospective adoptee seeking to enter the United States pursuant to the IR-4 or IH-4 visa classifications;

My mother already entered the US with her F4 immigrant visa and she has her passport stamped by the CBP, which means that her visa serves as a residency for 1 year before she gets her actual Green card. Does she qualify as an LPR? According to the USCIS website, i think she is:

Effective Date of Lawful Permanent Residence. A person is generally considered to be an LPR at the time USCIS approves the applicant's adjustment application or at the time the applicant enters and is admitted into the United States with an immigrant visa.

Also, I'm 17, so I'm a dependant child. My brother also has his immigrant visa stamped, so he is considered an LPR if the same rules apply. But he is over 21, so I can't use this exclusion in the Presidential proclamation:

(iv) any alien who is the sibling of a U.S. citizen or lawful permanent resident, provided that both are unmarried and under the age of 21;