The only difference between prop and lock is minutes played and number of runs per minute. There's no reason Merrin couldn't play prop, reduce his minutes and run the ball more frequently. We'd lose nothing by having him in the prop rotation.
Merrin has been playing less as a prop who is attempting to provide momentum, bending the line and softening the defence early in the set, and more as a smaller forward building on the props work by trying to find the gap in the opposition defensive line, using his larger frame (relative to a half) to hit the line, offload ball and extend the attacking play. just because a back rower is playing in the middle of the field doesn't mean he is instantly a prop. they do defend in the middle of the field with the two props and the acting half, so do get through a lot of work in defence, but their use in attack is different and builds on the more physical work of props. that's why props play less time and smaller middle forwards play more.
It's also possible to play with one prop and two locks on the field, although with too many blokes playing long minutes you waste the four players you have on the bench. The drawback is a lack of size, and the benefit is increased mobility.
that was Stuart's major blunder. He consistently selected a forward unit of smaller and mobile players than Queensland, and what you would even find at most clubs. he got extremely poor use out of his interchange because of it, and also, failed to realise that it doesn't matter if a player is generally a smaller forward who is used to playing more minutes (like Gallen & Bird who he selected at prop and the smaller middle forward respectively in game 1 2012), these guys need to be interchanged when playing in the middle with the amount of work required in both attack, and particularly defence if the team is losing the set count. this was another problem for NSW last season. the only way a mobile forward unit
may get on top of a bigger pack is if they dominate the set count and NSW didn't. this meant that these smaller forwards were absorbing more defensive pressure in the middle against QLD's bigger bodies and fresher prop rotation. QLD's fair dinkum props and Meninga's/Henry's effective rotation of them eventually wore down the NSW smaller fill-ins like Gallen, Watmough and Creagh, and the disproportionate game time they got from Stuart too.
You can also play with three props on the field and potentially another three on the bench. Plenty of size, but mobility suffers and you're f**ked if you get a few injuries - props are up there with (most) hookers for lack of versatility.
three props on the field is overkill and 6 in the unit is rare. you'd never see it. it'd end up making the team cumbersome and one dimensional, particularly if there isn't one or two that can hit the line hard and then offload. and as you say, it's a poor way of building a the squad. there has to be a balance of speed and size and strength. having that one smaller mobile body in the middle that can build on the momentum the props can generate provides more of an attacking threat. this is what Gallen and Merrin can do after their props have provided the platform for them earlier in the set.
problem for Merrin is if Gallen is selected as the smaller middle forward, (as he should be) the need for Merrin is reduced, particularly as Watmough is an automatic selection in the squad too. neither of those 3 should be considered as props, and all 3 cant play as smaller forwards because they play the same position. not necessarily the same way, but the same position.
our fair dinkum prop rotation to lay the platform and win the trench battle should be 4 of them from Tamou, Grant, Woods, Fifita, Mannah, etc. Props who are doing this every week and doing it very well. Origin is the last place to ask a player not regularly playing this role to step up and complete it. Give them 35-45 min of gametime each and manage the interchange properly. Most importantly, ensure Gallen sticks to the game plan and doesn't become the team.
i really hate playing a heap of players out of position. it's easy to shift a centre out onto the wing because he won't get in the way and it's just one position over, but when you start playing several guys who are used to a more prominent role at club and then expected to play a completely different role in Origin, the on-field structure eventually suffers when the game isn't unfolding perfectly. you see too many cooks in the kitchen so to speak and that is what has been happening for NSW far to often. I could imagine Pearce, Carney, Gidley, Merrin, Gallen, Bird, etc all on the field at the same time and eventually losing structure as they lapse into club role. This doesn't happen with QLD. They retain their structure and stick to their roles in every set and eventually wear down NSW and come up with the winning plays.