I can't recall the max rpm, but I could get up to slightly over 40MPH in top/fifth gear on a good day.
Farhad suggestion of the TPS could be good though I always got a fault code before replacing mine a couple of years back. I did post images somwhere of the track wear and lived with the fault...
It sounds like a limp mode because it is. I had times when a code didn't flag up and switching off and restarting (three times ?) clears whateve may or may not have been on the dasboard.
My guess would be an electrical issue, damp in the fusebox - or any one of a number of connectors seeing it...
I've not seen an Accord amplifier fail unless there has been a lightning strike in the vicinity.
If the loudspeakers have got damp (in which case there are options out there) then stick with the OEM version, which can be repaired.
Try fixing the seat first, whilst seemingly separate it might have something to do with the cruise control.
You are out of options as far as I can tell without spending far more time on the original issue. Are the seats independently wired for memory ?
Is/are the key fobs original or...
How many miles are you doing a week and are any of them long runs ?
What was the last MOT emission test result and is it much different from previous years ?
That is nuts.
I had a Mitsubishi Galant as a company car once, the dealer dropped a replacement sidelight bulb into the headflight before an MOT and it was off the road for over a week while they basically took the car apart to retrieve it.
I very much doubt there is a fault in the wiring loom. The control switch on the steering wheel controls two sets of filaments at the front of the bonnet (and both are on with the issue as described)
Have you changed/upgraded any bulbs, or had had a road accident lately ?