Gotta run home, but I'll answer some of that.
You state actions before initiative, so that people don't play:
"I won initiative! I attack at full OB!" or
"I lost initiative! I full parry!"
essentially that is the big one, but plenty of opportunity for actions conflicting, someone might be more inclined to declare it if they already knew they'd go before the person likely to stop/interfere/attack them.
Short actions also: Mount or dismount, instant spells, moving maneuvers that take less than 50% action. (Which covers a lot of different possible actions.)
I need to actually look at the cancel again, I don't want to say it wrong off the top of my head. . .but %10 action is -10 if you do another action. . .like if you cancel to attack, you're now limited to at most 90% activity to it, so at least a -10 to the attack.
I spent all sorts of time writing all those opportunity action examples, then we cut the round. . . .I liked those examples. . .grumble grumble. . .heheh. At least it came out in PDF, so that wasn't wasted effort.
Opportunity actions had a lot to do with the actual sequence. . .like, if you're a mage, and declared "I'll fry you with a lightningbolt if you draw that sword" technically in RM2 sequence, on M&M phase he could draw, then on melee phase he could stab you, and you'd not be able to cast. . . so opportunity actions let you "go out of phase" by holding a stated action and triggering it later.
My brain is fried, but I think there is a bonus to prepared actions in RMC AL allowing you to similarly sit on a declared action then pop it later. Beyond that, you can do any action at any time, so there is less need for a rule to allow out of sequence actions. . .there is also a "Wait" declaration, ala "Quick perception 10%".