As per IRCC guidelines job offer must be for at least 1 year along with other requirements which you can check in the link below.
https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=695&top=29
As long as your LMIA satisfies this requirements ,you should add it to your profile and claim extra 50 points.
Once you get an ITA, you can always extend your work permit (BOWP) if it expiring before a decision is being made by IRCC on your PR application.
Regarding what if you lose job question,
positive LMIA means you have the job secured, it never asked you to perform the job first and then claim point.
so no need to worry here.
hope this clears all your confusion.