Sorry, let me rephrase.
If one account signs address A and the other signs address B and then we ever see coins moving from A to B it's a good indication of alt accounts.
No.
Again, this is where technology will help. Like the plagiarism bot, a good algorithm can detect alt accounts from many years ago. Unlike the plagiarism bot, the results won't prove harm, and will need to be reviewed.
OK, even I'm lost on what you are asking on this one...
If user Alice signs wallet address 1aaaa...
and also signs wallet address 1bbbb... and then user Bob posts that wallet address, then it's a sure sign that Bob is Alice. If Bob is asked to sign wallet 1b... but can't then Bob is using Alice's wallet address to impersonate them. If Bob *can* sign 1bbbb... then Bob is Alice.
If user Alice signs wallet address 1aaaa... and Bob signs wallet address 1bbbb... then they could send Crypto to each other all day long and it doesn't prove anything except maybe they have satoshis to burn on TX fees.
IF on the other hand Alice signs wallet address 1aaaa... and Bob signs wallet address 1bbbb... and then BOTH wallets are in the send side of a TX - then that proves Alice and Bob are alts as they are using wallet addresses from the same wallet.
e.g.
tx 123456789
output
wallet address 1aaaa... BTC 0.999
wallet address 1bbbb... BTC 0.001
sends to wallet address 3cccc... BTC 1.0
The only way both wallet addresses can be a part of the outgoing transaction is if they are part of the same wallet.