With that in mind, I propose that the Roman Replacement army ability may only be used before to have an unengaged unit replace a unit that was engaged at the start of the turn. Neither unit may move or have moved this turn. I think this addresses all problems that have been identified and brings us to the original intent.
Any objections?
None here. The skirmishers thing should be dealt with, either by putting the official stamp Ajax's convincing interpretation of the rules that skirmishers don't technically "fail a rout check" when the auto-rout, though it's probably simpler all around if you just say that the 5" backup radius does not apply to skirmishers (or, as I'd prefer, say that it only applies to Roman non-skirmishers).
I'll do have one request, though. That these new Replacement rules (plus previous Triarii rulings, such as their back up giving +1 die to engaged attacks only, and the requirements that they have to be unengaged & on close to back other units up) be put on the official errata page.
I blame Kevin.
In fact, that's my new motto.
