I can see that logic for not having ghosts for destroyed segments. Guess I was thinking similar to what I've seen in vanilla where a destroyed item leaves a ghost that can be replaced by construction robots if you have the relevant item...doesn't really affect these though as the wall will respawn as long as the emitter isn't destroyed and you have enough power. And you're right, it's easier to tell where the gaps are without the ghost as they are very similar to the actual walls...I can't even begin to count the number of times I've thought a ghost was a real object. Having the preview when configuring it would be great though! I'm getting better at figuring out "distance" but there is still a lot of guessing and then if it doesn't sit exactly where I wanted it, I have to start over....particularly frustrating with the green walls which have a high respawn time. An alternative might be if there is a way to only respawn changed segments instead of the entire wall...for example if one were to change a wall section into a gate section take out that section and respawn the gate based on the current respawn rules, but leave the rest of that emitter's wall segments up. In addition to being a way of addressing my base reasoning, in theory your making those types of changes in "peacetime" and there is plenty of time to deal with the gap in defense, but I have also made on-the-fly changes like that before and a gap of a couple wall segments isn't nearly as bad as losing an entire segment of wall.
I'm not sure what you could do with blue off the top of my head either, but I like the list you put together for the rest. As for energy usage, I'm not sure how I feel about that one either. It is a good concept, but I tend to feel the high energy usage is the "cost of business" for using this type of wall and not sure I would do anything to lower it. Maybe make that the effect for the blue walls? Doesn't make the wall respawn or repair faster, but if you have an area where you need less power usage such as a small remote outpost with not much power generation, you can upgrade the blue walls to use less power...limited of course so it doesn't get too low.