I ride the trail all the time with road 28s. I did Duquesne all the way to DC on those tires this summer, no tire-related problems.
For parking in McKeesport, I've always parked for free under the bridge by the Marina and never had a problem. I don't know what the officiality of that is, but I see other people park there and load/unload bikes. You miss the Riverton bridge that way, though.
I think the loop is pretty well signed at this point. (That definitely was not the case in the past.) Last weekend, there was a brand new sign *explaining* the loop where it forked outbound from McKeesport, which I thought was a nice addition. (Alternatively, it's been there for a while, and I never noticed it.)
Basically, just follow the "YRT" signs - you'll start seeing them at the Marina. If you're facing the river, go left. You'll hook around behind what I think is a retirement home. You cross a road, and the trail is not-so-great in pieces along there, but some of it is brand-new asphalt. You turn right, go up a rise, and you come to the loop fork when you're at a bridge. For the left hand path that I'm recommending, uh, go left. That means going over the bridge. That's one of the few bridges where I use the sidewalk, because the signs say that bikes should do so. I ride down the sidewalk across the bridge, then stay on it as it bends left, and take it a short distance to the end (watch the bench!). At that point, there's a crossing to another trail section which is super nice brand new asphalt. That merges onto a road for a short section, then bends off to the right again for the one thing that resembles a hill that you'l encounter. From there to West Newton, you're on a really obvious trail, and you don't need to think too hard about which way to go. I think the rest of it is pretty obvious if you see the signs, but I'm not sure how well the signs show up in the dark, particularly the blue&white "YRT" signs.