Thanks for writing this up.
About a static split: I actually built a "fixed amount" mode for splitters a while ago, but when I played with it I didn't find a real use for it, so I took it out. I'll take another look at it tho, since you're not the first to ask. For now, the output limit on a Power Storage is the way to get a fixed number as you told.
The recycler: it needs power for every kg it works. If it gets less power than the trash coming in needs, the part it can't cover goes out the trash output unrecycled. So when you see trash coming out, that recycler is short on power. It also happens right after a power dip, because the trash that piled up during the dip comes out as trash once the power is back.
Light and heavy work the same way. With full power the Weight Sorter splits 50/50. The less power it gets, the more of it comes out heavy.
You can see both of these for yourself in a practice room. Open the Codex, go to Machines, and press TRY IT on the Recycler or the Weight Sorter.