Job Change
I will be writing from what little I know and have read but please do not take this for 100% certain, please consult a good lawyer for planning purposes.
Since you have not filed the 485 (and you currenltly cannot as you are from India) your options are somewhat limited.
If you change jobs your current LC based 140 could still be valid since it is based on a future job offer, however you will need staunch support from your current employer, and since you did a J1 waiver and you are thinking of leaving, I suspect that is not going to happen.
If you wish to start the process all over again with a new employer then there are mixed reports on whether you could retain the priority date if the previous employer cancels the 140. Some lawyers say once approved it is yours and others say the priority date can be substituted for a future 140 only if it is left standing when you request the transfer. However one good thing is nowadays you could get to the 140 completed stage very quickly with PERM and EB2 140 premium processing available. Technically you could do that PRIOR to leaving your current employer by working that out with your FUTURE employer.
If you move to a new job you will still have to maintain legal employment visa (most likely H1b status) and hence depend on your new employer to do that for you.
In regards to the NIW 140, of course you would have to continue to fulfil the terms of your 5yr obligation in HPSA/MUA for it to remain valid. Again you would still have to stay in H1b status as you have not filed the 485. I hope you self petitioned for the NIW140, if you did, I guess you could try to transfer even that PD to a future 140(but not too sure). You might also have to consider which of those Priority dates is earlier(NIW or LC based 140).
All in all quite a messy situation with a potential minefield all around. I guess it depends on how bad your job is and what your risk tolerance is in the situation.