There are a different ways to address this and they all benefit from insight into your problem space.
e.g. The reasons you are choosing POP over IMAP at all. Synchronizing mail across devices (your problem) is one of IMAP's differentiating features over IMAP. Lose your laptop, spin up a new laptop, connect via IMAP, voilà, all items in your sent items inbox,...
Google live person support offering have varied from time to time and the availability have depended on several factors. AFAIK several of them were not officially disclosed but people commented that some contact options were available for some but not for others.
The first thing to try is to follow the account recovery instructions in the Google sign in page....
Is it possible to get through to a person, explain that the phone
number on file is old and ask Google to authenticate using the
No. You will need to use Google's process for recovering access to your account.
You can access Google's Troubleshooter here: Can't sign in to your Google Account You may have to wait a number of days to regain ...
I feel your pain. If you nest a label one would assume (prefer) that the parent label inherit the child label, but as you've noted, it does not.
A common solution is to add another filter that catches all the same messages as combining the 'child' labels into a single filter would.
Mail Filter 1:
The domain alias is not the problem.
With Google the user account alias is the problem.
How Google choose to DKIM sign or not
For Google to add the DKIM signature the username must be one of the Google account aliases.
Say you have DomainA.com(primary) and DomainB.com(alias)
Your google account is my@DomainA.com
DKIM signature will be set when sending from ...
I found a solution that worked for me was to edit the contact, deleting the address you don't want on this list. Then save the contact, reopen and add the previously deleted email address. The formerly second-listed address will now be the first listed address and hence the default email address for any labels/groups the person is part of. But that will be ...