That PoS are called "coins" is a misnomer. What makes digital things "coins" is the real world energy that restricts their creation. Without mining, there is nothing that prevents duplicating PoS schemes endlessly, because doing that is free. With mining, the difference between Bitcoin and mineable altcoins stays clear because the meager demand of altcoins means meager price and less mining.
If it were true, if tomorrow someone invent a cheaper source of energy all PoW would be doomed. It's not the case because it's not the energy which restricts the creation of coins, it's the protocole rules. In PoW miners vote upon this rules, in PoS holder vote upon this rules. Not much difference, except that one can argue interests are better aligned with PoS.
No. As long as energy has any cost, the competitive situation in mining will lead to that the marginal return is zero (as is the case in every activity as per Economics 101). What exactly is the cost of energy is irrelevant, since all coins' difficulty adjustments are structured to adjust to a fixed number of coins per day regardless of the hash rate.
I am not going to argue more about why PoS does not work, because others have done it better, and you have not believed them. It is not the lack of good will and wishful thinking
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