1) no, Plod will often try and bully you with this, just plead not guilty, speed alone can be considered a due care issue but 90 in 70 is not a due care issue based on speed alone
2) Doubtful he could make it stick without being a traffic plod with vascar or some other speed detection equipment. So even if you admit to "around 90" they have only circumstantial. If it was a traffic car then they would have most likely offerred you a fixed penalty.
3) You can get up to 6 points for both offences so it is possible to get banned in theory, unlikeley in this case
I would plead not guilty to both, the due care will disolve as soon as plod realises he'll have to attend court and it won't stick anyway. The speeding charge is not backed by any evidence so that would probably not stick and even if it did it's a 3 pointer.
Ignore the motoring evangelists above they talk a load of old pony.