I would try and do Portmeirion and if you ran out of time and had to return straight to Conwy, you could see Caernarfon Castle on the second day - having done Snowdon in the morning. Although I have been into Caernarfon Castle, I think it looks most impressive from the outside. You will see a creek on the west side. If you cross the footbridge, you can get some nice photos of the castle with the water in the foreground. This is best in the afternoon with the sun behind you.
Please note that another impressive castle lies about 5 miles south of Portmeirion - at Harlech. http://www.harlech.com
When you go from Conwy to Llanberis, you have the possibility of 2 routes. One is the A55 which is an expressway as far as Bangor and is probably the way you would go to reach the mountain railway for 10am. The other way is via the Llanberis Pass - which you will see far below you on the north side as the train climbs the mountain. (They stop briefly for you to take photos - try and do this through an open window).
I think you may find it difficult to park in Llandudno for a restaurant - and I did not observe many in the town. Perhaps they all eat in the hotels? One person on this forum said they had wonderful Fish & Chips in Conwy. If you have not tried British Fish & Chips - give it a shot. The chips are what you call French Fries but much larger.
This link should show you the weather at the time you click it. Click PLAY arrow to project ahead for about a day. Friday is going to see a front move through with cooler fresher weather behind it
Try using this map to see the roads in the area. You can zoom down and drag on the yellow man to see street view.https://email@example.com,-4.0389563,69311m/data=!3m1!1e3!5m1!1e1