The FX models are more "flat bar road bikes." They have thinner tires and less shock absorption even than the more standard hybrids they derive from. So no, these aren't mountain bikes.
But I ride my touring bikes -- which have narrower tires even than these bikes -- on tow-paths all the time. These Treks have 700x35c tires, which are plenty of air for a gravel path.
I don't know about the wheels; they might be fine, but I don't personally trust low spoke-count wheels. I'd want a strong hand-built pair of 36-spoke touring wheels -- Mavic 520 rims, 14/15 DB spokes -- on any bike I was planning to take on bumby surfaces. But that might just be me.
I don't really think you need a mountain bike, or even fat knobbies, for a tow path. And once you're back on the pavement, the closer your bike is to a road bike the happier you'll be.