I don't know the (more specific) answers to your questions (best of luck on your research on it), but had some indirect comments and questions. Does your partner and you do a lot of backpacking together? If so, have you thought of making or using a double quilt at all? Especially if you are pairing a warmer sleeper (often men in general), with a colder sleeper (often women in general), a double quilt can be a great way to save (well more like pool) warmth in general and help out the colder sleeper. It's also flexible too. If you're going out in particularly cold weather, she can bring the bag along and use that as well, and thus be a lot warmer (assuming she is the colder sleeper). If it's warmer conditions, you could just bring the quilt for the both of you.
I don't have a lot of experience with this outside of home use yet, but i believe Roger has written some stuff on this, and it's a pretty common sense idea/concept anyways.
The only problem is that depending on your combined sizes, the quilt would need to be quite a bit larger (if you're both average sized, i would say at the very minimum 70 inches wide but preferably for maximum comfort 75 to 82 inches), and so heavier and bulkier as well, especially if using Apex.
The problem with this system is it may not leave you a good solo bag for just yourself. I'm doing the above, but giving my wife my former bag, and so if i go out on solo trips, i will just use that bag rather than the double quilt. I'm actually shorter than my wife by a bit, so it works out fine for me, but may be a deciding negative factor for you if you couldn't use your wife's bag for solo trips.