H1b To H3 conversion for education

[h1btoh4]

Both me and my husband are currently on H1B and my husband has an approved I140. I plan to apply for H4 COS to pursue graduate education.

When can I quit my job: on filing for COS or on getting the receipt?
How long is it currently taking for getting the receipt?
If my husband switches job while H4 COS is still pending, can my H4 COS be considered in premium? (His potential new employer will file for H1B transfer under premium processing?)