Jose:
You certainly don't need as much strength, either torsion or bending, out near the wing tips -- so you can probably get away with opening up the skins at that point. I would make triangular holes, with web that follow the ribs, and with an inch or two of skin left unmolested along the high point of the wing (that's your "spar").
As far as ribs and sanding, no matter what you do your ribs are going to "print through" -- the question is whether (a) you'll notice them at all, (b) you'll only notice them when sanding, or (c), you'll get everything as smooth as glass in your shop, paint, pat yourself on the back, then see everything print through when the temperature changes (look at a shiny black Corvett on a sunny day to see this effect -- unless it's been entirely disassembled and reassembled by a fanatic, you'll be able to see all the factory joints, and any crash repair that's ever been done).
If the kevlar makes the balsa significantly stiffer than when it's raw, you may not need stiffening ribs. If it doesn't, then you should consider stiffening ribs in the back as well as the front. I'd just go with one rib in the back, and maybe just one in the front. Unfortunately, the best way to tell is to put things together without the ribs, then see how much of a pain it is to sand the thing.
And don't paint the plane black.