I have been playing with hammock designs for a while now. I have found that a set length ridgeline is more of hindrance than a help in getting my hammock comfy.
My solution: I have a 1.5" webbing strap that I put between the trees and my hammock can be hung from that at precisely the best point for optimal sag regardless of how far apart the trees are. This eliminates the need for a structural ridgeline when trees at an optimal distance can't be found and gives me one heck of a line to use for getting out of the hammock and storing my pack / gear off the ground.
I like my ridgeline. Mainly for the reasons you listed.
I actually have not had any issues with mine. I think that as long as the distance between the trees is a couple feet longer than the ridgeline, you are fine. With my 10' ridgeline I have hung from trees 12-20 feet apart.
I think the only problem is with shorter lengths.
The way my ridgeline is set up, which may be the case with all/most, the amount of sag on the supports does not effect the ridgeline within reason. I start with my straps pulled tight when I get in. This also makes my ridgeline tight 80-90 deg. Over the course of the night given the straps stretch, I have ended up inches from the ground (probibly 5-10 inches lower) and with close to a 45 deg angle. The ridgeline has still been tight through this.
This may be different for different lengths.
I use two short pieces of webbing tied around the main line to keep it from slipping.
Are these short pieces tied 'round the main line to keep the hammock rope (tied to the main line also) from sliding past a "bump" created by the short piece? If this is what you're doing, seems like you'd have to tie the short pieces down pretty tightly. Are they hard to undo when you pack things up?
Perhaps I've got the wrong picture altogether.
Or perhaps there just isn't that much force pulling the hammock ropes towards the center when the hammock is occupied, and I'm just making this too complicated in my mind's eye!
I use a knot that unties easily when you pull on one end of the webbing and I do tie it very tight.
It is a slipped constrictor -- and it holds even when there's a fluffy dino in the hammock.