You want to use the linseed oil on every surface. You're trying to protect the wood and keep it from cupping. Don't worry, it'll weather juuuust fine. Water requirements are going to be a function of construction, as much as material in my experience. Using a single slab of wood or resin will keep the soil damper than multiple slats which will allow more air in. The resin probably keeps a bit more moisture in, but in my experience, you're going to be watering daily or every other day no matter what. Bushes and small trees take up a lot of water.

Think this was identified as European beech in another thread (just to let fall from the top)

Lovely bark, looks a bit like the bark of Xanthorhorea (Australian grass tree)however, excuse my spelling!

I have just started using/experimenting with the resin and by far am no expert, but I like the finished look you get. On the box, it says that one coat is like 50 coats of varnish. It is hard, and shiny and really gives it a polished look. Yes, it runs all over the place. You need to put something down like an old cookie sheet to catch the drips. Also, directions say to elevate it; I used old coffee cans turned upside down. Pour the resin all over the piece, take a small paint brush and pushed the resin to the edges and over. After several hours, check the edges for drips that are starting to harden. You will want to get those off before it sets completely....Sometimes tiny bubbles show up, and heat will break the bubbles, (hair dryer) then it become smooth as glass. It will make your tesserae look embedded in glass. Becky