Sorry Carpe diem, I'd do W2 the other way around start in the Glyncorrwg carpark (much nicer post ride food etc.), avoids the dull there and back at the beginning of the wall, and you get to finish on the Whites downhill which is my favourite. You just need to make sure you turn sharp left after finishing the wall downhill section.
I guess the only downside is the whites climb is a bit full on as a starter, but you do at least get it done with fresh legs.
As for tyres I was on a small block 8 last weekend without any major issues, but probably wouldn't recommend that! But don't go too heavy tacky as there is a lot of linking fire road.
I'd say 3 hours is a fast time especially for someone's first go, I'd leave 4 to enjoy the black loop on Whites etc.