The no more nails stuff I looked at and it seems like silicone caulk or some equivalent and that should be fine. by "ply" do you mean plywood? Plywood is a big no no for outdoors. even if primed and sealed it shrinks and swells with environmental changes and that can cause your grout to come off or your tess to pop off. If not sealed, yes it can suck the moisture out of your grout. Being in a covered area will extend the life of the piece, but your piece would be far better off indoors.

I hesitated to reply to this one, because mosaic on plywood outdoors is a bad idea, and from what you said, I take it that the mosaic has already been done, or at least a lot of it has. Is that correct? If there's anyway to transfer it to another substrate, then that's what you need to do. The outdoor sign I linked to below is done on Wedi board, which is weatherproof and does not warp. Then it was bolted to plywood, reason being that a plywood sign already existed and had the hanging mechanism already installed, so it worked out well to do it that way. But the tile mosaic was glued to Wedi, not plywood. I didn't try to get any tiles to stick to the edges of the Wedi; I covered the edges of the Wedi with thinset and then painted the edges of the Wedi and the plywood behind it with gloss Rustoleum enamel paint. The sign has held up extremely well.

Now, if you have already glued mosaic work onto plywood, the only advice I can give is to seal your grout with Dupont outdoor grout sealer - saturate it well - and paint the edges thoroughly, then go back and check the work every year or so, looking for loose pieces.

Since the work is on a wall (vertical surface) and not a horizontal surface (like a table or bench) the mosaic has a better chance of surviving. However, you need to just expect it to eventually fail or have to be repaired periodically.

I sealed a wood table with a marine sealant, top and bottom, then mosaiced it, the client left it outside, and the mosaic came off in sheets; I had to re-do the work on a piece of Hardiboard.

Here is a link that might be useful: Silva's blog, South Austin Music sign

yep, sad to say, you wrote the key word when you said "more porous" all the other choices in adhesives and grout and sealants won't matter. wood for outdoor mosaic is never good even in under cover, it will absorb the humidity and swell and degrade. I'm so sorry