I can only speak to #1 "Can it decrease the braking efficiency of the main levers?"
Yes, it might, but not a certainty though.
The amount of impact depends on the quality of your installation. Any join in the outer cable could allow compression, and cumulatively add toward sponginess in your brake lever. Also if the interrupter lever has a firm close point, or if it can flex while closed, when compressed by the brifter.
Another possible source of flex is how the lever holds the inner - some use a clamp, some loop the inner through, and some use two separate inner cables. This can also increase/decrease the cable pull depending on how the inner enters/exits the lever. I am not familiar with your suggested model, but it looks like there are two outer stops (with barrel adjusters) and the inner just goes straight through without clamping - this seems ideal.
If you used high quality outers, and make a good square cross-cut, and used good ferrules (assuming they fit in the receptacles) then the impact will be minimised. Make sure to tap the ferrules with a soft hammer, to seat them onto the outer before crimping. This reduces settling.
A possibility might be to use compressionless brake housing, even if only in the short length between brifter and interrupter levers.
Also, you could halve the potential issue by only fitting one interrupter lever rather than two - don't need to have both sides the same.