"...if I can take the trail almost all the way and then easily cut over. Is it possible to get off the trail near St. Nick's?"
Yes. If you're coming up the trail instead of crossing the 40th Street Bridge, at the end of Riverfront Park, you will turn left under an overpass and over some railroad tracks, then cross the road to get to Maryland Avenue.
"Am I going to get killed trying to do so?"
No, but you'll have to look both ways and run across the road at the "pedestrian crossing".
This may be further complicated by the fact that - last I checked - Maryland Avenue was closed. Don't know if you can get through on the sidewalk or not. If not, you would need to go to the next street, Stanton Avenue, and loop left up to Maryland via Sheridan Street.
"At north end of 40SB, cross PA28 and get on the sidewalk, turning right..."
Was over there about a week ago... alas, the whole area across 28 was torn up and inaccessible; sidewalk appeared to be gone. And that was the "official" access, obstructed and muddy as it was. That leaves only the aforementioned goatpath on the south side of 28, (which I personally love and find exhilarating, wicked "camber" and all ...my new word for the day!). ...Hope they plan on restoring access to the bridge from Maryland.
"At the stop sign/pedestrian crossing at the bottom of the hill, turn left, under bridge, to first street. (Probably easiest to cross the dirt area, rather than using the street)."
To be clear, the stop sign is not at the pedestrian crossing. The pedestrian crossing is at the dirt area, and yes, it is probably easiest to cross there. But ya know, I've gotten to the point where sometimes I just don't wanna.
Recently, all I wanted to do was turn left onto Grant Ave. (the main street), and I took advantage of the low traffic in my direction and got in the middle lane at the stop. Traffic was solid (two lanes) in the opposite direction, and the curbside driver was distracted, looking to their right, and began pulling out, on course to hit me, before looking back and braking. From that experience, I will be extra hesitant about crossing two lanes at a stop (though, offhand, I can't think of any others that I encounter).
IMO, this is a problem area, which needs to be addressed. The existing setup is inadequate. There is plenty of pedestrian and bike traffic here, but you pretty much have to fend for yourself. Even setting aside the 40th Street Bridge access (which is vital), here's a nice town with a nice riverfront park, and this is how you get to it? Likewise, it's not very welcoming to the town for trail users.