have approved 797 and in USA for 8 Years.
Can I go to India in March for Stamping.
Same time, Can spouse(h4) and kid(h4) can stay in USA? (They have approved 797)
If yes, How many weeks, can visit India?
Sure you can.
Yes this is fine. However if for some reason your visa is denied, they will need to leave the US asap.
There is no official guidance. I would suggest consulting your employer’s immigration lawyer. Though stay for few weeks should be totally fine.