It's still on the same power grid. The renewable energy would have been used one way or another. It's still contributing to the burning of fossil fuels on the grid.

I can't see why not. A government could require exchanges to prevent (or even just delay or tax) the sale of Bitcoins that originate from miners that cannot be shown to have used renewable energy to produce the block. The government could, for example, physically inspect the mining rig, certify it as renewable-powered, and require that the miner uses a particular keypair to sign their blocks in order to identify them as "green." Exchanges use this (government-issued) keypair to determine which Bitcoins are "green," and apply whatever restrictions are necessary on those that are not.

What do you mean? All transactions are verified by miners, so all transactions are equally "dirty".

So there is actually no such thing as "a bitcoin".

It's all just accounts with balances.

coin taint is a real thing but ultimately your wallet just looks at your inputs and writes outputs to other wallets.

There is a crypto called Monero that doesn't have this problem. it is growing fast.