The problem with taking the Border Princes isn't actually taking it, as I understand it. It's holding onto it when the inevitable Waaaghs of orcs come to visit.
That's what the line of superweapons I outlined in my plan is for.
You actually probably wouldn't need that many due to Greenskin psychology if Iron Rock is taken beforehand. One superweapon on Barak Varr and one on Iron Rock to make that both untakeable above ground, and then .... I think two huge bases in between them would be enough, each with a super weapon as well. Three if you want to be real certain that every prospective Waaagh would be close enough to a base that they'd be unable to resist attacking one.
Any Waaagh would have to travel from the BadLands between those two Dwarf Hold's to reach the Border Princes, and no self respecting Waaash, would sneak through the space between all of those targets without attacking attacking at least one of those four or five targets first and then, bam, superweapon.
Again, it's not the most practical of plans, and constructing those bases on the border between the Border Princes and the Badlands would be an actual nightmare, but once the bases and the superweapons are set up I think the line would hold against basically any Waaagh.
Though, no promises on the Skaven not getting upset about this shift in political power and digging to our base from below.
.....
Really, the biggest problem is that I don't think you could construct the bases before conquering most of the Border Princes, I don't think the political will to keep them staffed and funded long term would be there. So you'd need to conquer a big enough chunk of the Border Princes and build the bases and the superweapons without a Waaagh big enough to disrupt any of these bits long term coming through at any point in the process.
I think it might be possible, just not probable enough to be worth getting enough favors owed to us to gain the backing it would need and not probably enough to risk that many lives and that much money on a longshot this long that we are initiating.
We make plenty of big gambles, but we make them to reduce risks and dangers, this is a gamble that would introduce more risk and danger to more people than I feel comfortable with. It would be fun, but probably not a good idea.