No need to grain fill the veneer top, but any mahogany needs grain filling in order to get a smooth finish, including the neck and headstock.

No idea about the pickups. I'd be careful about removing covers on the kit pickups as I've no idea what they've used inside them. It may be standard wax (which shouldn't be a problem) or it may be a resin (which would probably mean damaging the coils to remove). And what goes for one type of kit pickup may not be the same for another type (e.g. if someone here has removed the covers on a full-sized humbucker). So if you are thinking about it, I'd definitely be prepared and have a pickup replacement plan ready. Those kit pickups aren't at all expensive units, but replacements will certainly cost more.

In the Gibson world, the Epiphone-style mini-humbuckers (LP Deluxe) with adjustable pole pieces aren't just thinner versions of standard humbuckers. The coil with the adjustable pole pieces is similar, but the other coil has a single steel bar rail slug. Like a standard humbucker, there is a bar magnet underneath.

The Firebird-style mini-humbuckers coils are different as each have a bar magnet where the slugs would go, with no magnet underneath, so are much more like a Fender coil construction.

Based on lowest cost manufacturing, I'd expect the kit mini-humbucker coils are probably two steel bar slugs with a ceramic magnet underneath - but I am only guessing.

What I don't know is how different a Ric pickup is compared to a Gibson one. I do know that Ric ones use thinner 44AWG wire compared to the standard 42AWG for Gibson and Fender, and Ric pickups are epoxy encapsulated rather than wax potted.

For general info, here's a good video on the difference between Gibson full-sized and the two types of mini-humbuckers they use: