Everyone has summed up Dora/Tai as not a great combo, Don't get me wrong though, it's not specifically because it's "The Lesbian and the only bi chick" issue,
It's because of what Dora is fast becoming, which as everyone elses viewpoint seems to agree with aswell as mine, She is now coming off as a vapid attention-seeker, pretty much regressing to the last time she was truly Bi, which, if my personality reading isn't skewed to hell, would indicate high school, so in short Dora is now grooming a teenagers personality in the middle of her 20's.. which is ridiculous.
That said, Tai having a romantic plot is long overdue properly after her whole sub-arc of trying to become Monogamous again, but Dora just isn't the person for this, Hell as impossible as it is due to it just being a ridiculous concept, but at this moment Tai/Marigold would seem more natural in my eyes.
As I said when the comic first started to seriously consider Tai/Dora, they just weren't/aren't compatible at this point in their lives. Dora is a self-made, reasonably successful young women in her mid to late 20s and if not for the Sven issue, would consider herself very successful. Tai is a college stoner who means well, is having issues of polyamorous and monogamy. Still a party girl, but looking to change.
They both want change, but vastly different kinds related to where they are in life. They are two people with massive relationship issues, Dora the worse, who should have waiting before jumping into a potentially disastrous affair.
You seem spot on with your connection that Dora is reverting to her HS days when she had girlfriends, possibly even her coven days (where she may have been equally self-absorbed). Being with Tai is already stunting the growth of therapy and bringing out an even worse side of Dora. The dress attire does make sense. Probably been a long time since she was with a girl and it was during a period of immaturity. It is definitely unsettling to a gal with her class, business success, and age reverting to this mess. And this is ONLY a day into the relationship. Imagine what subconscious stuff would come out down the line.
The Lesbian/Bi thing isn't even an issue at all. This is one problem with decent age differences. Different places in life and one or both sides trying to "age down" or "age up" to appease their partner. Just ughgh. A massive train wreck is coming. Not sure when, but it still come and I'm betting it'll be a worse kind of breakup than Marten/Dora if they let it last a reasonable amount of time.
Someone needs to nip this in the butt.

I'll be impressed if Jeph manages to pull this off believably because I just see no chemistry, connection, stability, and sanity (I get this is QC, so sanity is ALWAYS in question) but man oh man.