Gary D.
The US Forestry service tested various finishes and their results regarding protection mirror your approach. The most protective finishes are those that create a moisture barrier. Top was epoxy. Having worked on boats with epoxy I hesitate to use it on walnut, it is too hard to sand.
Penetrating epoxy is supposed (according to advertising copy) to soak into the wood and build a barrier in there. Again, I have not seen it used on walnut and its working and drying times are a bit of a puzzle. If it works as advertised it would be a good filler-sealer on which to use final coats of linseed oil.
