With IWB, you've got a LOT of extra retention going on with the belt across either the cylinder or the area just above it. In my opinion, a thumbreak is NOT needed.
Two clips or one? Depends. I recommend against any design that puts one clip "dead center" at the area of the cylinder. Basically, you end up with too much stuff "piled up at that high point" and comfort suffers.
With a 3" or 4" barrel, a single clip at around the triggerguard area or a bit behind can work, where the gun doesn't want to roll forward because the topstrap area encounters enough upper belt edge/pants lip tension to stay upright. With 5" 1911s or similar sized autos this works great because of the gun shape.
With a low-slung little snubby, esp. a hammerless model, a one-clip design of that type might allow it to roll forward. Ditto any very heavy gun - by the time you hit SP101 weight levels (26oz) I think you need two clips, one fore and one aft as shown below. With GP100s, L-frames, N-frames or bigger, you definately need two clips.
A lot depends on where you're mounting it, and your physical size. In other words, on a great big guy like me with a 48" belt (hey, it WAS 50, it's dropping!) and kidney carry, there's damned little tension across the topstrap and a one-rear-clip type might "roll". On a smaller guy, the belt will be doing a more pronounced curve at the topstrap and hold it up; ditto if the big guy shifts it to directly on the side carry versus back closer to the small of the back (but NEVER on the spine).
Double clips eliminate the roll. On a bigger gun, that means a fairly wide rig. Tucker solves this by mounting all the kydex to a flat piece of leather, which then curves around the body. My personal all-kydex equivelent is to hand-curve the carry rig to the body of the owner - on a big guy, dead flat can work but on a smaller dude, more curve needed.
This thing was a CHALLENGE:
That's a seriously cut-down 454 SRH in a double-clip IWB rig I built for Joe. Clips are wider than I usually do, and are "twisted outwards" so that the belt is guided around the gun in a less abrupt fashion than flat clips would have done. The forward clip is free-swivelling so that the rig "flexes" as you sit and stand - so far as I'm aware, I'm the only one doing that. My clips are .090" grade kydex, while the main body is .060" - relatively thin, but the overal comfort is there. Also, I do a single-side mold - if this was flipped over, you'd see it's basically flat where it contacts your body, versus "jutting in". Only myself and Tucker do that as far as I know.
