First, the cap FTF's: failing to fire on the first strike but then firing on the second strike is characteristic of a cap that's too small for the nipple cone; it does not seat properly and thus the pyrotechnic material does not feel the first hammer impact, which drives the cap into proper position and then the second strike is able to ignite the material. 3 solutions: 1) be sure to fully seat the caps (using a wood dowel), 2) use larger caps (not as easy as it sounds) or 3) sand/file the nipple cone down just a tiny bit to get a better cap fit.
By the way, nipples that have been peened by excessive dry firing also exhibit this characteristic, but I ruled that out as your gun is new. However, you might check the nipples to see if they have a tiny ridge at the top of the cone. If present it's easily removed with a file or sandpaper.
Nipple thread size: I believe you ended up with a Uberti; their midsize frame replicas use a 12x28 thread nipple, which is virtually identical with the quoted M5.5x0.9mm size (a thread which doesn't exist in the metric thread standards, by the way).