Sealcoat and the General Finishes High Performance is a good combo. Anything you put on the wood, whether it's a finish or mineral spirits will "darken" it. Even a simple wax finish on bare wood will bring out the color "darken it" to some extent. Though simple wax doesn't offer any protection to speak of.
Any polyurethane will add some yellow-ish/amber color over time, regardless if it's waterborne or solvent based. It's the nature of the beast.
If you want a water-white clear, non-yellowing finish then you'll need to look for an 100% acrylic based product. Acrylics will also be UV resistant (UV won't break the coating finish, readily), but that is only in regard to the finish itself. UV will bleach the wood color over time while the coating over the wood remains intact. If you want UV protection..cover the windows with a blind or install a UV blocking film on the glass. One easily available 100% acrylic finish is Minwax Polycrylic. In general, about every other Minwax product is..less than great. (their 2 part wood filler is fine for endgrain filling under paint). I've used Polycrylic on occasion, on medium to lighter woods. Their satin version is easy to apply, pretty forgiving in application as it levels well, and typically looks good. Waterbased lacquers are water white, but application may be more than you want to get into.
There's also acrylic-urethane products available, which, again will add a little color, though not as much as a 100% urethane product. GF High Performance is an acrylic-urethane. Good stuff.
Some comments on Sealcoat. Sealcoat is a 2-pound cut of shellac flakes. That's a fairly high-solids cut. Typically, shellac is used to heighten color and grain when applied over raw wood. It's also a sealer..like when it's applied over wood knots to prevent tanin coming through a topcoat(paint). When shellac is applied to raw wood, it's most often as a 1 pound cut. Shellac is dissolved in 95% pure ethanol (the remaining 5% is water and denaturants). If you use Sealcoat, dilute it 1:1 with good ethanol. A key point when using shellac. It's pretty difficult to coat a broad wood surface when applying it with a brush. The ethanol evaporates very quickly and if you don't keep a fast application pace and a wet edge, you'll get brush-edge overlaps. And the more you try to fix them, the worse they get. Fortunately, this is reversable as you just take a rag wet with ethanol and wipe the shellac off. Of course, this can leave a uniform, light coat of shellac on the surface..which may not be a bad thing. When I use shellac(often) I use spray equipment. It's easy to spray a uniform coating, but you need spray equipment. Sealcoat (shellac) is sold in spray cans. That may be a real option for you, Just don't spray it on too heavy or it'll sag. It's best to practice on some cardboard to get a feel for it.
Shellac is a terrific product. It's a universal conversion coating. If you want to use an oil based stain and water based topcoat..put a coat of shellac between the stain and the topcoat and it'll work. Want to glaze a piece once you've applied dye..put a coat of shellac on the dyed wood to seal it, You can then apply any glaze (or gel stain), wipe off as much as you want without impacting the base, dyed or stained, wood.
The are few, to no materials you can apply to raw wood to better bring out the color and/or grain than shellac. Oils will do the same, but not as well as shellac. Shellac also comes in many different colors, but that's a whole other topic. Sealcoat is blonde-dewaxed.
So, what to do? Shellac and High Performance should work well, but I'd spray the shellac on. And practice your whole finish plan technique before you dive into the real thing. It's better to burn up some material in practicing than wish you'd done that after your project is messed up.
(..and the apply shellac and sand when wet technique does work to fill in imperfections in the right cases. Not sure about fixing edges of veneer..practice first. )
