H1B cap-exempt to H1B cap-subject

I am currently on H1B cap-exempt (COS) working full time at company A. I have not left the country, so I do not have visa stamped. Last year, another company, B, filed cap-subject H1B on my behalf. It went under RFE but got approved recently. The new cap-subject H1B was files as consular notification rather than COS and as a part time. I will be working part time at company A (amend H1B) and also at company B. Now I need to head out of the country and get visa stamped. My question is:

When getting visa stamped, do I need to present both H1Bs? If not, which one should I present?When I return back to US, do I need to start working at company B right away?What happens if I quit with company B and convert full time in back in company A? Will I be able to re-use cap-subject H1B? Do I need to leave the country in that case?Thanks in advance.