dont ever risk a car on the beach around here - even a 4wd is iffy - this is tractor country and we have towed out shed loads of 4wds on the back of a tractor!
Knott end is fine given good weather. Conventional launching is ok here most of the time, the over the side method is only neccesary on a spring with some wind. Come on a neap tide and you can use the slip at any time. Spring tides can be a little more difficult.
Stanah is ok for an hour and a half either side of high tide on springs-but go 2 and a half hours before so that you can see the banks and the bits to avoid.
PM me if you are going out and we will help you out with info or bring a boat ourselves for a run- the pub on Piel does a good pie and a pint!
we have lots of clients who come back with their boats for a first time launch and we look after them first time out