What it sounds like you almost certainly want is a "hybrid" bike -- those tend to allow a more upright posture than either mountain bikes like yours or road bikes like mine (and drop handlebars are the curvy kind that I have -- I love them, because I like to minimize air resistance, go fast, and keep my center of gravity closer to the ground, but that's definitely a personal preference). That will incidentally be lighter, too, which I predict you will enjoy even if it's not strictly necessary. They tend to come with moderate-width tires with some tread but not big knobs; I think those are pretty good (less work to pedal than mountain bike tires), but it should be possible to change them out for fatter, treadier ones if you want.

Quick release skewers can actually be replaced with non-quick release ones pretty cheaply and easily. You can even get locking ones if you're worried about people with wrenches, but those are kind of pricey, and probably not worth it -- I've never had a wheel stolen even with quick-release wheels, though I don't tend to leave my bike outside for long periods.

I have heard good things about Bike Boom (http://www.bikeboom.net/) in Davis Square -- they sell good quality used bikes. I'd be happy to accompany you on a bike-shopping expedition if you want, to have an extra pair of eyes.