That's the core of the problem. You will not find much data for the resin uptake, because it varies more than 100%. No two pieces are the same. I have had balsa, wich was almost like birch, and I had balsa, wich was like xps foam. I have seen balsa wood with fine structure, and wood with a sponge like structure.
The small influence of the core to the bending characteristics results directly out of the principles of mechanics. And the pressure resistance of a sandwich is proportional to the pressure resistance of the pure core material. It's as easy as that. Have you ever had airex or rohacell in your hands? Maybe you picturing it like insulation eps. Real core foams are incredibly solid. You can drive your car across an airex board without damaging it.
I am not saying you should not use balsa. I am just saying that it seems to me that you are glorifying for some reason. And your image of it being an top end ultralight option ("as light as possible") and much stronger than foams doesn't correspond with my experience.
And resin uptake is a tricky beast itself. other than the used wood, it depends on your resin, the thickness, the surface preparation, infusion aiding preparation and many more.
If you want to seal it, use a quick setting epoxy, or a 2k laquer.