1) I can't see how a single socket with live/neutral swaped can explain
a "live" reading on the neutral at the electricity meter. What exactly
did he mean by "live"? I doubt both phase(live) and neutral could be
sitting at 240V. Nor should it be responsible for RCD trips(which
happens because the current on the phase and neutral is different -
i.e. leaking somewhere).

In his position I would have flicked off the main isolater and to see
if the problem cleared, then switched on the MCB's one by one.

Significant voltage on the household neutral (which is what I think he
meant) would suggest to me that it is poorly tied to the suppliers
neutral - and I'd want that fixed urgently.

2) Yes, if a faulty piece of equipment was plugged in. Don't use that
socket

3) Yes, there is something odd happening and needs professional
investigation.

4) Probably not a major expense, if it's an isolated stupid mistake by
a professional or an isolated case of uninformed tinkering. If however
it's the tip of the iceberg of a rewire by a madman, then very urgent
and somewhat expensive work may be required.