I use and really like gel stains and I use regular oil base stains, the stuff that has no body (like Minwax). HOWEVER I don't have experience using either for fretwork. I would think the gel stains would not work well with fret work because they are so thick and conventional stains should work well because they flow well. Generally I 'control' the color by wiping the stain off with a clean rag after some amount of time. So if I want a little color I wipe on a stain and wipe it off immediatly. If I want the color deeper I'll leave it on for few minutes before wiping it off. That would be tough to do with fretwork.
I always seal wood with shellac before staining. (3 oz. of unwaxed shellac flakes to 12 oz. of denatured alchohol). It's very thin. Goes on easy. Dries in a couple hours. I think for BB ply I'd use more than one coat because of the edges. I always do a very light sanding between coats and before any stain or topcoat.
I have use another product called McClosky's Sanding Sealer. It was fine.
My general practice with finishing is to make test pieces. I'm working on a vanity and have tried about 8 different stains/topcoat finishing combinations to get the color/finish I was happy with. It takes lots of time but after selecting or creating a plan, purchasing wood, milling, carefully cutting and/or joining - I'm not about to hurry the finishing. Here's a test piece.