is transferring to a part-time H1 more difficult to transferring to a full-time H1?
-No. No difference. Your lawyer should know it.
can a foreign national stay in the USA with a part-time H1?
-Yes. Partime H1 status = Full-time H1 status = H1 status. You can change back to full time h1 anytime just by filing an H1 amendment as long as the employer wants to that for you.
what is the minimal hours required per week on part-time H1?
-No. Not specific, of course if you can work > 20 hours/week, that is ideal for part-time H1 application.
