Good news: had some more interactions via twitter w/T-mo, and someone behind the scenes must have fixed things because I tried again this morning and the activation worked! Added to my existing plan with the "double your data" offer.
Moral of the story: If you have issues and a support rep tries to say you need to get a t-mobile sim instead of an Apple sim, it is NOT true!
UPDATE: One other thing they called back to clarify, and I'm glad they did because it is a bit confusing: they have two similar sounding data plans (amongst several other options) for the ipad air 2:
First, there is a *prepaid* 5gb pass for 5 months for $10. If you get that you can use up to 5gb of full speed data (in addition to the free 250mb a month you ALWAYS get). It's a one-time payment of $10 for a one-time allocation of data that gives you that pool of 5gb that lasts for up to 5 months.
Second, if you're a postpaid customer you can "double your data" and match your phone data allocation on your tablet for $10 *per month*, up to a max of 5gb a month. Since I have their unlimited data plan I end up getting 5gb per month for $10 per month.
I also checked, and just as with their phone plans selected music services (e.g. rdio, which I use a lot) do not count against data at all, so for me that 5gb a month will be more than enough. In essence I'm getting more data than I'll probably ever use per month for $10 a month, which is a great deal, IMO.