I'm getting laughing, but I'm not getting counter arguments from you.
We have, respectively: The one who both can fight 9-tails and has the power of 9-tails.
The one who can subdue 1-tails and crush small armies with a single jutsu
And the one who got captured by Kurotsuchi with a little help and really hasn't shown all that much high-level capabilities aside from Edo (which is specifically noted with an *).
What's the problem?
People keep going "OMG Kushina's so high!"
Then I go, "Hey, she's strong enough to fight Kyuubi even without it's power, that's rather uber."
Then they don't give a reply to that...
then awhile later I get "OMG Kushina's so high!"
C'mon, looping while not answering the main point in her favor isn't cool, and I'm not going to change ratings if people complain but don't have a counter argument.
Bee wasn't that surprised. He immediately realized that Naruto could not do it because he was not on friendly terms with the Kyuubi.
Even with a decade or so, Kushina still did not reach Naruto's level (claiming the chakra of the Bijuu), so she's out. So I doubt she could use the Bijuu ball willingly.
Mito is harder to determine. At the very least, she likely had free access to the Kyuubi's chakra like Naruto does, and at best, her mastery was akin to Yugito's, i.e. access to full form & Bijuu ball, but not full mastery like Bee or Yagura.
But the only thing Naruto seems to be lacking for full mastery at Bee's level is the Biju's cooperation. The full taking of the chakra as well as cooperation can't be the requirements for Biju ball unless Bee and Yagura are the only two people ever to do it, which seems unlikely.
We know physical transformation (as opposed to cloak) doesn't require high mastery too- we saw that with Killerbee, who could do limbs even as a kid.
I think Yugito's mastery is lower than Naruto's and more akin to his earlier transformation, just with more control. Naruto's is higher in that he can access more power, it's just he did it in a way that also cut off transformations that he earlier had access to.
Also I will note Kushina had Kyuubi much better sealed than Naruto did (likely because she could refresh the seal at any point, and tie it up in internal combat). So where Naruto could retain full control at 3-tails... she could likely do several more.
I get the impression the normal way to do it is to work up control tail by tail.