Willboy I see no problem with running Yorkshire 3 weeks before. If you are fit enough to be doing 3.45ish at Yorks and then take it easy with maybe a few gentle runs in between you should be more than capable of finishing at Snowdonia.
RC, JB is spot on with the parking. There is a good amount of parking space in Llanberis but remember this is the towns busiest weekend so get there early and save yourself the stress. The traffic wardens show no mercy! Oh and watch out for speed cameras if you are driving down the pass through Nant Peris. The van is sure to be there that weekend. It nabbed Mrs LG while we were there a few weeks ago for the trail marathon.
Congestion up the pass isn't a problem. The field soon thins out.
The idea of sitting in the lake has never crossed my mind for a moment. I can see why it might help recovery but it is the end of October and normally pouring down so perhaps not the most attractive option to most! Still, each to their own.
TR22, that is a great first marathon time. If you are in the same shape come October the sub 3.30 is definitely doable and maybe faster. There are some really fast sections to help you bank a bit of time before the last climb.
The longest continuous run I have managed was 38 and 52.4 in a day as two marathons with a 2 hour break in between. I was dead on my feet both times. Neither required any degree of navigation otherwise I would probably still be out there somewhere. One day perhaps TR but slogging through the small hours after having spent all day on your feet is something I will leave to you and Brer.