My thoughts and experiences…
.22 rifles/uppers/conversions-
We have one of our drills shooters that comes out and shoots his .22, and for a lot of what we do it’s a viable alternative. However, for nights where we’re doing hammers and triples, speed reloads, etc. the .22 is a detriment. The bolt doesn’t lock back so speed reloads are not done the same way. The minimal recoil makes working on hammers a joke since it’s nowhere near the same as firing 5.56 doing the same drill. The only time you really see a use for the .22 is in trigger control with accurate, sighted fire, and even then it could be debated as even doing controlled pairs is different with the reaction time between shots being faster with the .22
9mm rifles/uppers-
If you go upper only, the mag block becomes an issue since none of the bottom-load mag blocks are of any decent quality, and few if any of them activate the bolt hold-open. The best of the blocks, the Hahn top-load requires removing the bolt catch, which effectively means you need a dedicated 9mm firearm, not just a lower. The 9mm can actually have more recoil than the 5.56 which means that again hammers and triples, and any other fast shooting may not translate back to the 5.56. While the bolt may lock back on an empty mag, depending on the block used, the magazine is a completely different geometry to reloads are not really the same. 9mm also has a reputation for being finicky and hard on internals, so the cost of replacing these parts and the time spent getting the gun to run should be included in your calculations. and finally, magazines are an issue, and it would appear that C-Products may now be the OEM producer for Colt, which would be a shame since their magazines contribute a lot to the finicky reputation and reliability issues.
5.45 rifles/uppers-
To truly see any appreciable savings you need to go with the corrosive ammo, otherwise you (almost) may as well be shooting Wolf or Barnaul 5.56. This presents all sorts of obvious problems with regards to cleaning of the gun. At something like a 3-day training class away from home you may have limited access to things like a garden hose and/or an air compressor, which would be my preferred way of cleaning the gun. I suppose a bathtub and loaner hair dryer from the hotel may suffice in a pinch.
The other issue here, like the 9mm, is that C-products is pretty much sole-source for the magazines. Any time I have to rely on them for something, I get nervous. Some people appear to be able to get the 5.45 to run with 5.56 magazines, others not so much.
Finally, and across all calibers, remember to factor in the cost of mimicing your primary setup. If your primary is an SBR with an S&B Short Dot and a rail system holding a Surefire Scout, mimicing that in the alternate caliber can get extremely expensive. Then factor in alternate magazines as well, and I think that at least 10 is the minimum there. Sure, you can go simpler or use knockoffs since it’s not your primary, but the less you copy your primary the less the benefit of the training. Plus, the use of knockoffs like the SPOT on anything other than a .22 may lead to reliability issues.
Overall, I think that the 5.45 is the most viable if they become readily available again. I wouldn’t use it on the same lower as my primary though due to possible corrosion issues and hammer spring issues and would build a dedicated gun. I would limit the mimicry to sight/optics, grip, stock, and sling and not worry so much about light and rail system, and just use the primary in any low-light shoots. Even the 5.45 Silver Bear is about $.26/round vs. the 5.56 which is $.33/round (Aim Surplus prices for both). I’m not sure who has the corrosive stuff in stock right now.