If theyʻre just little screw holes, Iʻd fill them with Plastic Wood or anything similar. Or sometimes I just drive a new screw in and leave it there.
there are areas that are basically unfinished where the "U" wood slats used to be - do I need to treat these sections before I treat the entire area?
Try a very light touch of experimental Minwood stain. (Walnut is to dark, Mahogany is too red, as youʻll discover). Try colored pens or markers. Try furniture nick kits from a hardware store. Whatever works to blend the tones. Varnish coat will help blend.
I got the Interlux satin varnish, sandpapers of appropriate grain, tack cloths, and such - but do I need to strip the finish that's on the wood already? And to those with trained eyes, is the finish here likely some sort of teak oil or is this also old varnish?
No need to strip unless itʻs peeling or something. Iʻd clean it first with household cleaner. Then a quick sanding to get any loose stuff off and slightly roughen the surface. Five-minute job. Then a coat or two of satin, which is very forgiving and will make the compartment uniform in color.
When I revised my wine compartment on the 38, I discovered that the finish had been applied with a rag. The varnish was very dark, suggesting it had stain in it. The back sides of the rack components were not finished at all. Ericson was a quality builder, but hidden interior parts got the down and dirty treatment. Thatʻs my attitude, too.
Hey, the coffee maker is gonna cover most of this anyhow.