Some thoughts to add to your interesting ideas - without having read the article by Dr. Sims - but first of, great arguments and illustrations!
Remember that
Roman combat style did not rely exclusively on the
gladius.
A salve of
pila would tear through unarmored ranks. It's quite effective enough against armored foes.
Roman battles tended, according to new analysis, to involve a lot of time spent simply standing apart from the enemy, occasionally rushing on, and peppering the enemy with
pila passed on from the rear ranks; nobody can wield a sword for the few hours most battles took in one go, never mind a heavier
falx (though you'd have to convince the falx-wielder not to rush head-first and try to under-run the
pila). If the enemy stood far enough apart to dodge incoming missiles, they'd have to
face several legionaries all by themselves, and would be a tempting target to cavalry, should they be able to manoeuvre past their own ranks. If they stood more compact, the
pila would likely do their
work. Also note that according to Vegetius (who does write rather late, in the 4th century, but harkens back to the "
good old times"), legionaries were trained with slings. They would have contingents of ranged troops with them. Falx-users may have made rather nice targets if you could somehow avoid hitting your own people.
Second, the
shield itself was used as a weapon. The scuta were large, and while hitting someone with the boss is the most effective offensive use, angling the
shield upwards to hit the enemy with the lower edge can also do some damage and keep an enemy at a distance. It's risky, but it's shown on some (mostly gladiatorial) representations. I don't know what the length of a
falx is, and whether such a tactic would break the attack enough for the helmet to protect the
head, so it might be better to keep your
shield high. Also, while
Roman shield rims were metal-reinforced, I wonder what happened if a
falx broke through. How often would it get stuck? Even if it gets stuck for only a second before it
comes lose again, a well-trained
legionary would have a
good opportunity to strike at the unprotected belly of
his opponent, who probably needs both
hands to pull out
his weapon.
The Dacian may have been nimble on account of
his armour, but the
Romans might try to compensate by stabbing rather than slashing. The
gladius does both effectively, but stabbing seems to have been preferred. It takes less energy, is less predictable than a wide swing, and causes deep wounds.
Roman tactics could vary, of course. Take Arrian's Battle Order against the Alans for the use of legions against unconventional forces. Unfortunately, much of this depends on knowing the proportion of
falx wielders to other warriors - and as your
coin shows, there was other equipment - and so far as I know, no written sources on the
falx exist. Art representations, sometimes done by sculptors who
had never really seen action (there's cute scale-mail-pyjama wearing horses on Trajan's
Column - although the Adamclissi
Trophy sculptor would be hard put
not to have come into contact with
Dacians, seeing as the
trophy is in
Dacia) and of course experimental archaeology are our main ways of testing any hypothesis. There might possibly be some way to use comparative
history, with respect two-handed/bastard swords or some shorter polearms, though I guess discussion on how close such similarities are would soon degenerate (and end up in the dreaded "samurai katana vs.
medieval plate armour" debate).
Incidentally: you might get some very interesting feedback here at the RAT forums.
