Well basically you got the idea right...
lemme see if I got this right...
you're intending to remove stance breaker at the beginning to get helm splitter for AS LONG as such enemy has
the off-guard status on it?
well if that were the case I'd probably add a battle-long event that checks the enemy status per turn
so you can learn and forget the skills instantly (it's a theory)
I'd probably do:
1. create a new placeholder status (call it Pain... or whatever you like) have this status be triggered once off-guard is out, (this status serves no purpose just to time your skills)
3. set the event that checks the enemy's status, basically when the enemy is about to lose it's [Off-Guard] status it replaces it with [Pain] when this happens have it instantly remove your [Helm-Splitter] and restore your [Stance Breaker] finally remove the recovering status effect off the enemy
(like I said I'm only using this status as a skill retrieval phase)
now moving on to the battle...
1. Player Turn = uses [Stance Breaker] Enemy is inflicted with [Off-Guard] status player gains [Helm-Splitter] AND forgets [Stance Breaker] (you can do this by assigning a common event to your skill)
1.5 NOTE: at this point you'd have the event set so that when the enemy loses it's [Off-Guard] to trigger the
skill swap.
2. Enemy turn = uses [attack (opportunity to counter, attack, heal etc)] Enemy has 50% DEF
(this stays the same

)
3. Player turn = uses [Helm Splitter]: Enemy is hit. (btw... you CAN make the helm splitter deal extra damage if you add [Off-Guard] as a weakness to all enemies)
and MOST IMPORTANT: make sure the attack removes [Off-Guard] and adds [Pain] or whatever you called it.
3.5 NOTE: At this point since the enemy is triggered with Pain status, this makes it so you instantly lose your [Helm Splitter] and recovers your [Stance-Breaker], also make sure this event removes the [Pain] status and resets it to normal to repeat the cycle.
by the way... I'm sorry for not properly testing this out since I don't have RPGXP installed on this PC... so basically this is all theory but should work.
good luck, and pretty cool combo idea you got there.