I've never used Shell Island - so I can't comment on the slip there. We are also heading for Neaps this weekend, so most of the slipways round here are going to be limited (until you get to Pwllheli).
However, there are some caravan/camp sites between Shell Island and Barmouth that launch off the beach, one is Caerdaniel (there are at least two 7m Ospreys that launch from there) - so you could possibly stay there?
Alternately there is Barmouth Harbour. The slip is also only accessible around High tide, but if you are comfortable recovering on the beach within the harbour, then you can do that there.
You could launch Friday afternoon, and put it on a mooring in the harbour for the weekend. There are several not being used at the moment!! You may be able to do the same up at Shell Island.
Your best bet would be to talk to the Harbour Master at Barmouth (Glyn) or his assistant (Asa) on 01341 280671.
Hope this helps!!
P.S. You also need to be registered with Gwynedd Council to launch - need to show proof of insurance etc. Ignore this if you already know......!!