The fastest way for them to get a PR would be to file for I-130 (Petition for Alien Relative) and I-486 (Adjustment of Status) concurrently/together. To be able to file I-130 and I-485 concurrently, your parents need to be physically present in the US, and the entire process - assuming there are no RFE's - takes about 6 months to complete, and they will get their PR card within this time. For this to happen, you will need to bring them here on a B1/B2 as you've stated. Going thru this process will not be considered a Visa fraud - however, what attorneys advise is, while entering the US on a B1/B2 their intent should be to return back to Nepal after their authorized stay in the US, and this is what they should mention to the Customs & Border Protection/USCIS Officer at the port of entry upon asked. However, people change their mind all the time, and if they change their mind after entering the US and decide to file for I-130 and I-485 for a PR status, you can do so for them.

2nd option of filing for them while they are in Nepal takes longer - about 10-12 months (assuming there are no RFE's). Going this route, you can only file I-130 and when this gets approved, which usually takes about 6 months, the case is transferred to National Visa Center (NVC) from USCIS. From this point on, all communications will occur between you and NVC (USCIS will not be involved). NVC sends you some forms to complete, asks for additional/detailed information about you and your parents, and asks for some documentations from your parents. Upon providing all these info/documentations - only after they have specifically asked - they will require your parents to take a medical tests at a designated clinic in KTM and schedule the immigrant visa interview at the embassy in KTM. Like mentioned, all this process takes about 10-12 months.

And you do NOT need an immigration attorney to file for your parents - it is very simple and self-explanatory if you go thru the instructions for I-130 on USCIS website.

Hope this helps. Good Luck.