No, this is not an abuse case. This scenario is missing some steps.
After you swap, you would first have to vendor some items, craft them to a point where you could use them a bit. Level up your MG rank and gain a bunch of favor. Then buy all the items you want as though you were MG all along but took a massive detour and wasted a ton of time as CoF.
If the implied last step of this scenario is then to swap back to CoF, you end up swapping back and putting your original gear back on.
So the potential for abuse here is negative as this would put you right back where you started with no gain and a ton of wasted time.
Edit: I was mistaken that the final step was to shatter the purchased items for use in CoF and while this is possible, it’s still not an abuse case because you could just get those same crafting materials as CoF while still utilizing the other CoF benefits and not eating up time gaining MG favor with a lesser gearset.