Cap won't have artifacts unless the loop is too close to the edge - in that case not only you'll get them after removing the loop, but also because it affects cage projection on this the edge. So there is some planning involved, but it's the easy and fast (you can store the loops in an Edit Poly) if you can't add more…
I think it depends, the loop helps a lot with the waviness, from horrible all-round to usable (but still present). You can get better results depending on the loop placement (too close to the edge and you get artifacts like yours since it affects the edge bake). Triangulation shouldn't matter since there are hard edges and…
You can also add a support loop on the low poly, right over the inset, bake with it, then remove it for the final low-poly mesh - it will hold the cage better and get rid of some waviness.
Hi. It not really possible to get a normalmap without wavyness with such lowpoly cylinder. But there are some workarounds. This is what I do in the cases when its possible to apply it: Make the lowpoly, and duplicate it. Instead of adding support loops and subdivision modifier, use chamfers so the circular segmentation…