I've almost no experience with acrylic sprays, so can't really comment, but the Colorspec 2k clear coat info doesn't give it as suitable for wood (though they may just have thought it wouldn't be used on wood so didn't list it). But I know others have used acrylic clears on necks, so it might be OK. It shouldn't sink much into a maple neck. Or you may need to apply a normal 1K acrylic coat first. I just don't know.

The ash will need pore filling a sit's an open-pore wood. The maple neck won't. Timbermate is the pore filler that's most commonly used here, but being water-based, it can shrink, especially if you don't leave it fora few weeks to shrink naturally before sanding back and continuing to paint. But I'm not sure what other pore fillers would work well. As you are painting it, you could try a UV curing resin to seal the wood as that shouldn't shrink - plus it only takes a few minutes in sunlight to harden fully.

The end of the neck should ideally match the curve of the neck pocket, so if there's a big gap as it's only touching at the corners, I'd certainly work on the end of the neck with a file. I've used chalk on the pocket walls before. Push the neck in, and where there's chalk, it's touching, so you need to file off those bits. Repeat until you've got chalk over most of the end of the neck. You'll never get a perfect fit (unless CNC machinery was used to rout out both the neck and the pocket, and the Chinese factories don't use CNC on the kits). As it's a 22-fret neck, it's got that overhang which will hide any small gaps - which will be between the scratchplate and the neck, rather than the body and neck.

The original 'clear' nitro finish of the 50s and early 60s was slightly amber in tint, and yellowed more with exposure to UV (many of the first 'TV yellow' LP Jrs had a white base coat, it was the lacquer that made them look yellow). Modern clear nitrocellulose lacquer is still very slightly amber tinted, but far less so than the earlier stuff and far more stable under UV.

So the best way to get an 'aged clear' finish is to use a light amber tinted lacquer, followed by some clear coats for protection (you could just use tinted amber for all the coats but it would probably end up too dark/yellow). Tinted amber is easy to obtain in spray can nitro, but not in acrylic. You might fine somewhere that will make up a light amber tinted clear for you (I know a lot of Supercheap stores have a custom paint mixing service - but I don't know if that applies to clears), or if you have a spray gun, you can get tints for acrylic lacquers that you add yourself.