There are two zippers on the main compartment so you should be able to lock them together.
The Elite Survival Systems Assault Systems Rifle Cases that I'd recommend are size #2 (33"). They are 8" in height which will allow enough room to fit optics and accessories.
The Elite Survival Systems Assault Systems Rifle Case size #4 has an internal length of 37" and internal height of 8.5". All Sizes larger than #4 have an internal height of 10.5". So for your rifle I recommend using a Size #5.
Yes. Measure your AR10 from front to back and choose the appropriate size rifle bag. Each bag is measured by it internal length. all sizes above 37" offer 10.5" of internal height to accommodate optics. Size 37" and below offer 8.5" of height to fit scopes and red dots.
The Elite Survival Systems Assault Systems Rifle Cases that are longer than 37" offer 10.5" in internal height. If your rifle needs 12" then this case will not fit your rifle.
Size 2 or 3 would work for your application.
Yes, it is designed for the FN FAL., so is the size 7.
These are internal lengths.
Elite suggested the 28 inch length. According to Elite, the magazine holders are designed for AK-47 magazines and should hold the 10 round 12 GA. magazine. They also suggested looking at their Bullpup case as well at the following link - https://www.opticsplanet.com/elite-survival-systems-covert-operations-discreet-case-for-bullpup-rifles.html
The size 1 is for AK-47 size magazines and the size 3 is sized for 9 mm magazines.