We always treated that 50% much like concentration for maintaining a spell, it drains you 50% activity, but no eye closing lack of focus for half a round in which you drop out of OB/DB melee posture.
We always considered the whole round as one declaration period, so for us the "Declare" then "Roll Initiative" then "Resolve" applied to hasted people and parry.
If there's only one initiative for the round in which the hasted person acts, and they declare the OB/DB split before initiative is determined, then they have to stick to one OB/DB split for the whole round.
Of course, that made us use a 200% activity round much like Ecth, we were never much happy with the "act 100% with the normal people, then get another 100% at the end of the round" scenario as it seemed to make the hasted act longer instead of faster, which seemed goofy to us. If you're moving twice as fast, then you should move twice as fast, not twice as long.
The worst street legal abuse of haste and kinda TWC I can recall is "I throw the dagger at him at full OB, then Full Parry for 100% action full DB with the sword in my other hand, then draw another dagger." makes it rather hard to hit you while risking a nasty thrown dagger shot, snap thrown and off hand penalty or not, since the sword DB would kick in right after initiative. I can't recall how that got stepped on.