Most of my guitars have been Limba for the last few years. I have a lot of experience finishing it.
This is how you do it. Since you've already grain filled your way its too late to tell you that's not the way to do it but whatever, keep going anyway. I would have used an oil based grain filler that dries in a couple days. Linseed oil never really dries enough to put a hard finish over it. I would have waited at least two weeks before finishing over it or not used it at all. Its too late now but down the road you might have delimitation.
Anyway, stop sanding in between coats, you don't need to do that. The only reason to sand in between coats is if you have some crap in there you need to get out. Don't wet sand it either. Get some dry sand 400 or 600 and use that. Plow the lacquer on there, don't be shy. Don't get all worried about a thin finish, especially on Korina. By time you sand it flat and buff it out it'll be plenty thin. Get enough lacquer on there so you can sand the finish perfectly flat with no shiny spots anywhere. If you sand through that finish you'll sand through the shellac and the oil. You will see it, it'll never have the same color even after you spray over it. If you think its getting close to sanding through go back and plow another 4 to 6 coats on it. Again, dont be shy, get it on there WET. I spray it right out of the can without even thinning it. You wanna get it on there. It needs more than you think it does. Start levelling under the pick guard so if you go through you can cover it up. That way you'll know how thin the finish is on the rest of the guitar. If you have to spray more to avoid sanding through do it. Again, don't be shy with it, lay it on WET. Once you get it perfectly level with 400 or 600. Honestly, I start with 220, spray a couple coats then level again with 800. Its faster but you might sand through without experience.
Then thin your lacquer out last least half and half and spray enough coats to cover the scratches from the sand paper. I use almost all thinner and just enough lacquer to fill in the sanding scratches. Let it sit for a least 2 weeks. Now is the time to wet sand. If you've done it right you can start at 1500 to level it out. Then buff it and hope you don't rub through the finish.
This is not an easy thing to do, even after 25 years of doing it I still sand through. Rarely does a finish go off without a hitch. It still gives me fits.
If you need some help send me a private message and Ill send you my phone number. You can call and I'll walk you through it.