I agree mostly with Kobal, the only thing that might have a 3D collision model are missiles(a fast unit that starts moving after a shot was fired can evade it).
And the twohander animation really is a block animation, I have never seen a twohander actually push another guy back. Since 1.2 most twohanders also seem to use only one stabbing animation against cavalry, making them more effective, however, their often really low attack stats are what makes them ineffective since the shield bug was fixed. I increased all twohander and halberd attack stats to numbers between 15 and 24 or so and that makes them quite lethal, giving german zweihanders more defense(that melee-only stat) seemed to show a lot more blocking animations which leads me to think the animations are the result of calculations, which is also the only thing that makes sense. Most onehanded units do in fact have a very high defense, which also explains their effectiveness, for a guy with 10 attack it can take a while until he gets a kill against 21 defense, however, if you give that guy 23 attack...
This was obvious to me when twohanded knights with hammers would always lose against their sword and shield counterparts, if the twohander has only 5 attack to break 21 defense while the sword guy has 13 attack to break the 6 defense of the twohander, the result should be obvious. This was a bit different before the shield bug was fixed and animations do apparently play into attack speed(faster animations = more attacks per minute possible) but if one tweaks the stats accordingly, there is no need to edit any animations IMO.(that's with 1.2 where the twohander animations are fixed).

Let me underline my point with a quote from a CA dev as given in this interesting interview:
The second area would be balancing, and that's purely because we shipped a game with 21 factions and close to 400 units. That's a ludicrous amount of stats to tune, and so more time would translate to better results. It's something we've examined again with Kingdoms.