Currently on H1B. Basically my question is, -> is there a expiry date for I-140. After getting first I-140 approved, what if I leave the country and come back later on H1 - can I still reclaim the old PD if I start the GC process again ? What if I switch to F1 for couple of years and then get back on H1?
Priority dates for approved I-140s in the first three preference categories may generally be retained for subsequent I-140 petitions. Consult with an attorney for more information.
You really need to pay an attorney to get the answers to the questions you are requesting. Specifically, hire an attorney or speak to the one representing your employer. You are dealing with rather complex matters.
Even if an attorney does give you an answer it will be with a disclaimer. Accordingly, you will have no assurances to the accuracy of the opinion.
Legal disclaimer: Mr. Zapata's response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Zapata's statement above does not create an attorney/client relationship.
I think you need to be clear that the rules on labor certification and temporary work visas are absolutely not created to be either convenient or flexible. The series of "what if" questions you pose are not likely to be workable in any part of the employment immigration system I know of. Learn the rules first, then begin developing your plans and expectations.