Went 84 with this hand in blanks. tried to walk 6s and got set because an opponent
had two 6s with one being the 6:5 and held them until I started the parade of 6s.

What are the odds that an opponent will have the 6/5 plus another 6?

It's just under 35%. (That also includes having all 3 others)
Of course they would have to hold on to them. But with only the trumps,
followed by the 6-6 since no other doubles to pull anything, most opponents
would. So, Richard Hay had a 65% "guaranteed" probability of making 84 on this hand.

Opponent bid 84 (but should have bid 1 mark they were up 6-5)

They got set because my partner 2 trumped the bidder in 4s with one of the 2 trumps being the 6:4

You will make 84 on that just over 65% of the time, so close to 2 out of 3 times.
Here is one way to break it down:
Partner has 64: 33.3333%
Left Opponent has 64 and no other 4's: 15.9649%
Right Opponent has 64 and no other 4's: 15.9649%
Add those together to get 65.2632%. This is therefore the odds of making 84.

Or to go from the setting side, you have:
Left Opponent with all three 4s: 2.6316%
Right Opponent with all three 4s: 2.6316%
Left Opponent with 6:4 and one other 4: 14.7368%
Right Opponent with 6:4 and one other 4: 14.7368%
Add those together to get 34.7368%, which is the compliment of the 65.2632% and is the odds that you will go set.

The statement about having the top two trumps statistically does have a lot of merit. In fact, with just the top two,
you can get past the trump trouble on 84 at almost 64%. So, that's nearly as good as this scenario.
But if you have five trumps with the double but without the cow, it goes up to 80%.
So having the top two is not always necessary for the odds to be in your favor.
Finally, in this case, had it been the 4-3 instead of the 4-5 (that is, not having
the cow or the calf but still having the double and 3 more), it goes down to 50.5%.
That's because an opponent with any two or more of the others will set you. But even
that's good to know if you find yourself down 6-1 or so, and knowing that you have basically
a coin flip to make 2 marks to possibly start climbing back into the game.