" linseed oil" Boiled linseed oil (AKA 'BLO') hopefully. Linseed oil takes years to dry, the boiled stuff is faster, but takes many thin layers. 'Once a day for a week, once a week for a month, once a month for a year, once a year forever.' Either tends to create a very dark surface (nearly black in some cases) as they cure by oxidation and polymerization.

Did the cabinet side swell and blow out the woodgrain? The cabinet side is probably flakeboard. That's sawdust and glue. Once it swells, you can't "unswell" it. If you know who made it, you might get the part previously mentioned. But you would need to remove the swelled part so the veneer piece sat flat against the cabinet side. Probably easier to replace the cabinet. Borg base cabinets run about $ 100.00 or so. The doors can be used over. Ron

It appear's the water got beneath finish and stained wood. Repair is challenging for a beginner. Complete removal of finish,bleach water stain from wood,touch-up color to blend then apply x coats of clear finish. I can't tell how top intersect's with front,rear and sides but laminating might be possible. Actually,laminating is possible,it's only a question of how much effort and skill would be required. A cover like you have on it is what most people do.

I can't give you answers to all of your questions, but I can answer a couple. 1. That vertical crack looks like a crack in the foundation caused by settling. It happens in a lot of older homes and I don't think it is super serious, but it could settle more over time, so you might have to re-do a repair at some future point due to additional movement. 2. That looks like normal cracking in old plaster...not an indication of anything worse. 3. The black stain is not mold. It is stain damage caused by water sitting on top of the oak parquet, either from watering plants sloppily and not cleaning up, or from a leak coming through the wall from outside. There are various ways to try to clean this up (just Google), but in my experience it's not possible to make them disappear completely as it's not just on the surface, but has penetrated down into the wood a bit. I once had to deal with this and finally used a power sander to physically remove the stains, and then refinished the surface to match the surrounding area as much as possible. I probably had to remove 1/16" of oak to get to undamaged wood. 4. How did you get debris behind a baseboard in the first place? In any case, the easiest thing would be to pry it off, clean behind it, and reinstall....and maybe cauld the top edge with latex, paintable caulk. 5. If I were repairing that wall, which looks like concrete with patches of spackle or a plaster skim coat. I would sand only enough to remove anything that is loose, and then use a 12" wide joint knife to flatten the surface with joint compound.