The only other formable nation close to Ming is Malaya, and you need to colonize for that. On the other hand, infinite ADM might be worth the wait.
Even so, the fact that coring time is still instant with a nation that has +100% coring costs - Dai Viet, for example - would suggest that it's bugged out in some way. It would be interesting to see if the same thing applies if there's another 100% increase in cost from Aristocracy.
It only applies if the costs are additive BEFORE applying to the cost and time. I did some math the other night because of this and came up with a theory. Let's say a province takes 4 years to core and has a base tax of 6, but is part of Dai Viet:
First, we apply the reductions(which are additive):
4 years + [(-100% + -25%) * 4 years] =
4 years + [-125% * 4 years] =
4 years + -5 years =
-1 year!
Then we apply the increases(I assume these would also be additive):
-1 year + (100% * -1 year) =
-1 year + -1 year =
-2 years!
To prove my point, this also works when applying to cost as seen in DDRJake's stream. He states that there is a hard-coded limit of 2 ADM per base tax.
6 tax * 36 (non-accepted culture) = 216
First, we apply the reductions(which are additive):
216 + [(-100% + -25%) * 216] =
216 + [-125% * 216] =
216 + -270 =
-54!
But, that is less than the hard cap, so it resets it to 6*2 = 12 ADM.
Then we apply the increases(I assume these would also be additive):
12 + (100% * 12) =
12 + 12 =
24!
Oddly, there seems to be no such hard limit on time, but this seems to be the answer. Hope I made sense