a) Yes, we can use approved I140 to get H1 + extension with another employer as long as current I140 is not revoked.
b) It depends on the new employer. If its a solid company and no risk then yes, they can file H1 for you. But if you are planning to stay with new employer then you would need to file PERM, I140 and port old PD. Else when date is current, I485 can be filed with old employer and when GC comes through you would need to join the old employer.
c) Your current job on H1 is not related to the future GC job as long as you are working in the same field. And H1, LCA would need to filed every time there are drastic changes to job. And similarly when GC comes through, as long as they agree to pay you the prevailing wage or greater than it as per PERM and you continue to work with at least similar duties in same field as mentioned in the PERM, it should be ok.