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.

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

A photo of the bark and of the entire tree would be very helpful.

Looks like worms or larvae. Got a use for faux larvae? maybe you could slice it, make patterns with the slices, then embed in resin. note: microwaving it does NOT turn it into amber. I know, I tried. It foams up and gets very, very messy. Uh, I was about 12 when I tried it, not recently.