Babies flip during labor too! It's more common the more children you've had. It sounds like you have a great plan and resources (chiro, Spinning Babies). And you have 6 weeks to go, right? Don't stress and just do everything you can do.
And please just don't forget, babies turn in labor too!!! My third turned from a standard presentation to a posterior forehead presentation when I was about 7 cm dilated. So, for me it was negative that my baby turned mid-labor, but if the baby is already transverse, the position honestly can't get any worse! At least a breech baby will come out!
Also, if you've had a kid or kids already, a subsequent baby will often not settle into birth position until labor actually starts. So you have that on your side. I mean, worse case scenario, you go into spontaneous labor with a transverse baby, baby doesn't turn in labor, and you *truly* need a c-section. At that point, you know you've done ABSOLUTELY EVERYTHING to give your baby a chance to turn.
One last thought, often times babies who present transverse or breech at *labor* have other issues going on. Issues that may not be a problem in utero, but will be a problem once they are earthside. An unusual presentation is often a signal that a baby will need extra care or attention at the birth or soon after. It could run the gamut from the baby needs a simple chiro adjustment or their frenum snipped to nurse properly, to having jaundice or needing a little extra oxygen at birth.
I'm sure your midwives have mentioned all these possibilities though!