Heh, compiling all your questions would be a good start for a Setsuka FAQ.
22A ~ BT B+K:B is guaranteed if 22A is a counterhit.
(It seems to be a rather useless combo. 22A is dangerous to use, especially if you want a counterhit... 4A and 6K are better for fishing counterhits. And they both deal more damage with their respective combos.)
66B+K ~ BT B+K:B only works if they keep holding the block button or if they tech backwards or to Setsuka's right. So most of the time it won't hit.
2) Regarding B+K:B, do you need the B+K part to hit the opponent before the :B will come out?
Nope. You only need a contact for 1A : A : A
3) Any tips to doing B+K:B accurately?
It's difficult.
For starters, you should probably start getting the ordinary timing down... I suggest that you do A+K,A, wait, and perform BT B+K:B.
It should help you understand when you need to hit B... (Pretty much at the moment that the first strike hits the opponent.)
When you get it somewhat consistently, you can begin with 22A ~ BT B+K:B.
It's harder to execute, because it's possible to input BT B+K while Setsuka is not yet ready to attack.
It means that you're doing it during her buffer window...
Since BT B+K won't start until she's ready, you have to wait a little longer until you can do the ":B" part, or else it will be too soon.
For this reason, simple muscle memory might fail you.
...So, you need to either get the proper timing down for 22A ~ BT B+K (Wait until the buffer is gone), or you can learn an improper timing which let your input BT B+K during the buffer and requires that you input the last B later than usual.
For the improper timing, you can use visual cues to see when to input B, but it's not recommended. Muscle memory is better in the long run. It's more useful online, and it's more useful if you ever play with a different TV... (Like at a tournament or such.)
Learning the proper timing is probably much more consistent.