Are there downsides to cross-top brake levers? Compatible with all mechanical-hydraulic disc brakes?

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.

I am a maker of custom bikes for kids and small adults. I am a heavy user of the interrupter-style levers, even for main brake levers. The main reason that I use them is that they are easy to adapt to a custom bracket which fits smaller-diameter handlebars and smaller hands. Another great benefit, they are some of the lightest levers available, period, at as little as 45 grams each in stock configuration. I am familiar with both Tektro and various clones.

To answer your questions,

1. It is not likely to decrease the braking efficiency of the main levers. I doubt you will notice a difference. Technically there could be some more friction from the cable flowing through the lever, but the lever itself should have less compression than brake housing. To minimize friction, consider using lined end caps such as those available from Jagwire here: https://jagwire.com/products/small-parts/lined-end-caps
2. These types of levers are "road pull", so they should be compatible with your Hy/RD road calipers
3. Tektro is a safe option but the various clones available on the market seem almost identical in function and even in appearance. Do be advised that many of them come in strictly left-hand and right-hand versions, according to the shape of the lever itself. This could be important if you plan to put the lever in an odd location, or in a "reversed" configuration.