Lynne, A few thoughts:
Sidemount rigs are very idiosyncratic, and you may have to pass through a very awkward and uncomfortable period before you get yours dialed in.
I would agree with the 'awkward and uncomfortable period'. I expected the BM to SM transition to be easier. Part of the awkwardness, however, may have been changing from the 'known' BP/W configuration, where my doubles were assembled, I had established the proper position of the tank bands, and the hose routing, weighting, gearing up sequence, etc were all worked out, to a new situation. In hindsight, the 'awkward and uncomfortable period' existed for BM, but was spread out over a longer time. If my first foray into doubles had been with SM, it probably wouldn't have been any more noticeable than it was originally with BM.
Sidemount loses you the ability to access all of your gas, if you have a first or second stage malfunction or hose blow.
I guess, in theory, you still have the option of moving a reg from one tank to the other after that gas supply in that tank is exhausted. So, technically, you still have access to all of the gas, in an emergency situation. It is certainly not as easy as manifolded doubles, obviously.
Sidemount setups are a bit more of a pain to get into, as compared with backmount, where you spend the time assembling the setup, but once you have done that, you shrug in and are good to go.
Yes, at first, and this is one of the things that I found I did not initially like about SM (in reference to your 'Other side' thread), in addition to the challenge of accessing my thigh pockets. Over time, I found that the amount of time I spent assembling my BM rig - attaching the wing and BP, attaching the regs, connecting the inflator hoses, etc. - wasn't all that different from the time I spent setting up the SM tanks, attaching the regs, etc., and I could more easily don my Nomad and then clip my tanks on and attach the inflator hoses as fast or faster than slipping into my BM rig. I initially spent a bit more time carrying the rigged SM tanks individually to the water's edge, and then clipping them on after I was standing in the water - I futzed around a lot more at first. But, with practice, I found that SM wasn't more of a pain, rather I was just more used to the BM set-up routine.
I have this suspicion, too, that you have to do significant rerigging for different sorts of tanks.
Possibly. It may depend on how you set up your SM tanks. I don't use stage rigging kits, nor do I use screw-tightened metal bands for the bottom points of attachment. Instead, I use tank cam bands (2", not the DR 1.5") for the bottom, and bolt snaps attached to 'pigtails' - loops of nylon braided cord - for both top and bottom attachment. For the top, I just slip the loop of cord over the tank valve, then attach the reg. For the bottom, the pigtails run through a tri-glide on each cam band, held in place by the knot tied in the pigtail to form the loop and tension on the band. I can easily move the top loops and cam bands from tank to tank between dives if I choose. In addition, I can quickly change the position of the bottom cam band on a particular tank - move it up or down, or rotate it to adjust the orientation of the valve opening by fixing the bottom point of attachment - on the surface before splashing or even underwater during a dive.