Excellent review and playing, Dave! Sara, too, you have a keeper there!!
As for the position of the lyre box and screw, I usually pick up my horn by the upper branch or the hand guard because it is usually sitting in a euphonium stand in the upright position. However, when pulling the horn out of a case, I most always pick the horn up with my left hand, and I, like you, use the area where the lyre box is on the ACB Doubler, and you are quite right, the screw sticks out a bit and is in the way. I can't remember where most of those lyre boxes are on other euphoniums, to be honest. Adams does not have one (no marching in parades with my Adams for me I guess unless I have the music memorized). There is a lyre box on my B&H Imperial, and I think, without getting it out, it is on the tubing coming out of the 3rd valve, so it does not get in the way. And the screw does not stick out, if I remember correctly. It would probably be pretty easy to relocate the lyre box on the Doubler to the 3rd valve/slide area. And have the box on the inside of the tubing with the screw hole such that it doesn't stick out.
Like this:
Agreed on the 4th valve lock. I owned a Miraphone M5050 like Rick, and the lock was real similar. But sort of hard to work it one handed, like you can on the Bessons, for instance. And I also think the M5050 and Doubler locks are probably overall better. But the easiest one is my current Adams. It takes absolutely no effort to use. More than likely due to the fact that there isn't a 4th valve lock on it! I worried about that initially (like when putting it in the case), but after 3 years, hasn't been a problem so far.