OK, I have a couple of unexpected “free” minutes, so I’ll address the way you learned the gemara.
The problem is, you’re reading something into the beraisa which simply isn’t there; the beraisa is referring to regular yibum, and to learn that it’s talking about multiple yevamos, with the objective to eliminate all but one, is pure speculation.
The Tur would also be completely out of context, as I already mentioned, and there’s no reason for him to expect us to understand his case to be that of multiple yevamos when the beraisa makes no hint of that.
Also, according to you, it’s perfectly acceptable to marry someone quite a bit younger, so why would age be a criterion for elimination?
One more thing – why does the beraisa say the issue is kettotah if the issue is finances?
The pshat I mentioned is the simple pshat, and it does not pose any of these difficulties.