How can "they" prove I (ever) did?
And if proven, how can "they" tell how many I own(ed)?
And after "they" proved I owned x amount, how are "they" going to tax me?
The same way "they" prove and tax cash transactions.
By voluntarily keeping records then.
Backtracking the blockchain to calculate how much btc I "owned" (at a(ny) given date) doesn't seem viable. Let alone proving the "ownership" of privately mined btc.
If I know a key to a wallet, do I own the bitcoins in it?
Possession is nine-tenths of the law, so yes. Unless someone else accuses you of stealing the key, if you know it then you legally own the bitcoins.
How do "they" know I have this key? Even if the wallet can be traced to me, they won't know how much in it is mine to spend without the key.
So the "Rob me" billboard analogy, as mentioned above, sounds plausible when the law dictates I have to surrender my keys for "audit" purposes.
Are the numbers I am manipulating with this key, really mine? To tax? Tax what?
Yes. To tax the money, of course, because that's what those numbers are, as sure as the numbers in your bank account are money.
I don't agree. btc is not money. Not unless my government accepts (tax)payments in btc it isn't. Until then it's more like gold and trading gold (as a service) is exempt of taxes. Only when you liquidise your gold you have to pay taxes (in a nutshell)
Until then btc should be considered LETS-currency, maybe.
Also, some of the btc I spent came from Satoshidice, which is a game of chance. Dutch law dictates you have to pay (income-) taxes over the net amount gained (wins minus losses) to retrace this (as a third party) is a next to impossible task. (to prove)
The numbers in my bankaccount aren't money, the stuff the atm spits out is. Or at least in my point of view.