on the SW corner of 34th st at seventh avenue there is a LIRR entrance, with a long escalator that takes you down directly to the LIRR ticket concourse level (which is one level below Amtrak/NJ Transit) or 2 levels below the street
but you still have another flight of stairs down to the platform-on some platforms there are escalators running up, but i don't ever remember seeing them run down!
and its embarassing to say i can't tell you where there are elevators- but i know i've seen handicapped symbols around
i will be in Penn Station tomorrow evening and will do a look-see and report back
but with all the shlepping- did you consider a door-to-door car service- and avoid Jamaica all-together?