For the draining - Use a Straw. Put it in place and then great stuff the pot to the back ground leaving the end of the straw exposed. Trim it back once the GS is all dried and you will have yourself a hole for water to train out without exposing the foam.

I think your tank looks great so far!

For the silicon I just wore rubber gloves and smeared it all over the back. Once you put the background against it and it dries you won't be able to tell if it was even or not, because the background will block all the light.