Lucky you! I see you live in NC; you'll be heading up to Pisgah in no time
From your description, yes you'll be looking at mtbs in the 13-15" range. As you know test-riding is key.
Several thoughts on the TT issue:
On road bikes TT length is more important than standover (IMHO). Standover becomes a bit more important off-road; especially if you are new and especially if you will be riding off-road. The more technical the trail, the more grateful you'll be for that ability to hop right off the bike!
Luckily, most modern mtbs have a sloping top tube.
Yes the effective TT length is a bit longer, but the seat tube angles are more relaxed as well. I think you'll be able to feel this when you test ride.
You can run a shorter stem on your mtb (I like a 110 mm on the road, but will run 100 or 90mm off road). You can also get handle bars with more sweep and even a bit of rise if you prefer.
If you just can NOT get comfortable on a stock WSD mtb, there's always custom