Organisation is excellent. I cannot really comment re the congestion at the start although I've don't recall reading any complaints (given your Chester time I wouldn't start near the back). Quite a few car parks (more so than Conwy!), but they are pay as you display. Don't get there too late - get one near registration - closer to race time the car parks fill up & I've seen lots of cars parking on grass verges - all of which get ticketed (traffic wardens usually out in force!!)
Sitting down in the lake or in the stream near a car park is a pretty close thing to an ice bath. After the pounding the legs get (particularly on the final descent into Llanberis) it's a good idea
Definitely achievable Willboy. Probably not much point doing much running after Yorkshire. Going to Snowdonia with the aim of just enjoying it is sensible. I am sure you will enjoy it and I wouldn't be surprised if you run Snowdonia quicker than you thought you would.
It sounds as if you will be better trained than I was the 1st time I did Snowdonia TR22. Yes, the hilly long runs will help and yes a sensible pace early doors is important. I wish I'd have had that advice before my 1st one - got totally carried away with myself - going through 10 miles quicker than I'd run 10 miles before (although not having done any long runs of more than 2 1/4 hours in training....)
TR. I wouldn't get worked up re the hills. The Llanberis Pass climb is pretty steady & isn't really steep - there is then quite a long descent which gives your legs the chance to recover. The Beddgelert climb just after 1/2 way catches quite a few people out - I'd say it's more of a drag than a climb - more often there's a slight tail wind to assist. The final climb is pretty much a struggle which many runners (including those near the front) walk parts of!
Speaking to people over the years, the general consensus is that runners should add about 15 minutes to their flat marathon time to estimate their Snowdonia time. I'd say for quicker runners it's nearer 10 minutes. Sub 3h30m should be well within your capabilities if you ran 3h4m on your 1st marathon
I've had a message from Brooks to say that it's 12 weeks till race day! I'd best start training!? I've only done one event this year, although that was the Trail Marathon Wales - quality over quantity!?!