As for void insurance, that’s utter rubbish. Even no MOT doesn’t affect your ins as long as the accident wasn’t caused by a fault that would have been picked up
It should be and it certainly used to be rubbish back in the days when your insurance went through an underwriter, there were no comparison sites etc etc. Now the model is different. The checking of what you have said on your proposal comes after a claim, and increasingly insurers look for reasons not to pay ( to quote SWMBO who is a Fellow of the Insurance Institute). One such would be the vehicle not being roadworthy and that includes issues like illegal plates, exhaust etc. Not just things relevant to the accident.
Depends on the company. Some are better than others so all I am saying is that you cannot be as certain as that.
As for the number plate,
the relevant rules are here . I'm not a lawyer and in any case I suspect the police have better things to do that to check plates unless you "show the wrong attitude when stopped ). That said the regs refer to both a plate and a number and the plate is specified as flat. So unless your car has been made before the specified date, you are obliged to have a flat number plate
