It's not normal to 'grain fill' as maple has closed pores, as does the rest of the ply the kit body is built with. That's not to say the surface will be absolutely smooth, but technically there are no pores to fill. The mahogany body on Ozbike's kit is open pored, so that did need grain filling. What you'll benefit from is really a 'surface sealer'.

CA itself can be used as a finish, GluBoost promote their range of CA products as such https://gluboost.com/.

Beware that using CA on a large area can create a lot of very unpleasant fumes, so is best applied outside in small areas at a time.

You can get a very shiny finish without using CA, but CA is less prone to sinking in after application than many other finishes so will stay even once sanded. But like any finish, if theres not enough depth to it, you'll find it hard to sand it level without breaking through the finish.

A couple of applications of thin CA should seal the surface well enough to stop anything else applied on top from sinking into the wood, but it does mean that you will need to stain the finish first, as it will be locked-in once the CA goes on and you won't stain the wood through it. I wouldn't try to sand the thin CA flat at all after this as it won't be deep enough, just a light sanding to remove the most protuberant bits before using your main finish.

So you need to ensure your stain goes on well, which means doing all the normal checks for glue spots. One option is not to stain the wood at all, but use a tinted lacquer spray to provide transparent colour to the guitar (as Ozbike did to hide the glue spots on his bass). This is also useful for hiding the glue line where the veneer halves meet and which is very difficult to disguise or remove using a glue remover. The visibility of this line will vary from kit to kit.