About the output resistance of a voltage amplifier
1st question


You are just getting confused. A voltage source with zero output resistance is an infinite current source across zero ohms. You can't just do a conversion from a voltage source with finite output resistance to a finite current source with zero parallel impedance because that makes no sense.


  2nd question


If the output resistance is very great you CANNOT say it behaves like any amplifier because you are only talking about one component. Ditto for the 2nd part of your 2nd question.


  Extra bit (mine)


I see you have asked 10 questions now and got reasonably good answers to several of them but you haven't formally accepted any answers yet. I would also conclude that you probably may not not have upvoted any answers yet. Now here's the deal with stack exchange. It runs on a form of currency, call it good-will if you want, and that good-will is upvotes and answer acceptance.

So, please take the tour and do your kind duty.