I recently had an issue with them and actually managed to get past the bot and have a direct chat with Harry, an apparently human agent. So it is possible but it was rather tedious. I could not find any alternative means of contact.
You keep the old passport, nothing is physically done to it until the replacement arrives. Yes it is cancelled in the UK "system" when the application reaches there but they do not tell anybody unless they are specifically asked. So whilst it is true that is is not officially valid for travel nobody outside of the UK will know that, so it's a risk you may or may not wish to take.
I do this all the time. The cable itself does not affect the charging rate, the charger does that and in any case will not charge something faster than it can accept, at least that seems to be the case with my devices.
The need to exit (or is that not the same as "stamp out"?) is clear. I would imagine the need to re-enter will be determined by then guards/officials at the border.
I have travelled in and out of Thailand many times with similar meds (inc. Bisoprolol) and have never had any problem. Having said that nobody has ever asked or noticed that I have them, either in carry on or checked baggage. I have never taken any paperwork with them, as these are over the counter meds I would think not needed at least from Thailands perspective.
As for the US I have once or twice taken them in and out but again never been asked.
You might want to look at this thread, I also think you cannot open an account there. What is actually difficult about transferring money that you need to "make easy"?
There does appear to be a fundamental misunderstanding by at least one person of the purpose of a multi entry tourist visa. It is to allow a tourist to base themselves in Thailand and make multiple trips out/back to other countries as part of their holiday. Those trips may be several days or weeks and the in between time in Thailand the same.
It is not intended to be used to stay as long as possible in Thailand before having to leave and return on a new entry, there are other visas/extensions far better suited to that.