Sam normally you do a pretty good of rules intervention, but here you are way off base. Not only are you ingnoring the RAW and adding in stipulations that are not in the book, but
you have just made using power weapons way more complicated than it needs to be.
Your reasoning, MFA! An interesting concept, most often associated with modeling size and height for advantage, but is now being applied to valid weapon swaps. Giving a space marine sgt a combi-melta is modelling for advantage, but this is part of the hobby and allowed. To get a combi melta on say a terminator sgt I have to scratch build one, or take one from another model. Both of which by these rulings are now no longer valid in Plainville tournaments. Neither came with the original model and the second actually came in a different kit. How are we proposing to solve this?
We're looking at having to put together matrixes of what GW model has what weapon and what it must look like. Does that not begin to suggest that you are overthinking this?
If the rules provide the option (that nice expensive book we bought), then it is up to the player to represent this in a wysiwyg fashion, if you stick to that simple rule, you will save a hell of a lot of time and effort.
I'm done for now, I'll be playing RAW, my Black Templar castellean on a bike with an axe won't be apologizing for anything, and nor will the one in terminator armour, the bretheren using Space wolf power weapons will also stand tall, all 2 inches
As for DCA (i don't play them though they are cool models) having an axe and sword is cheesy, but its legal. I'll mock you if we play and you have that but I'll not lose sleep over it.