Proof of connection, not only based on the same Address and POA or linked to Alt 1,2&3 accounts.
In the same case, the suggestion from @examplens, your way of proving, that you are innocent, that's the best thing you can do.
For your questions below, there are answers and back questions that you must answer.
1. They could use a newbie account to post of POA with the same telegram address and username that I have used in other bounty campaigns.
cases like this can be proven in another way, if a beginner does it.
1. What if the rookie is already working on another bounty and he receives a token with the same address as Alt 1,2&3, a different campaign, before they are caught cheating in the same bounty, Whether such a case can be said Alt, or brother, falsified or indisputable.
2. They could have to take entry in the spreadsheet with that false information too.
This is the same case as number one, the best solution you do.
1. Notify the Bounty manager, the rookie faked a spreadsheet and used your address and you created a topic in Reputation for abusing yours and warned him on a neutral profile, you're safe.
3. Someone could copy my POA info and post them unconsciously by not changing them. Sometimes hunters get limited time to join a campaign because of the limited available spot so they could try to copy the previous person's POA info and change them to join faster. This type of human error could be done.
As long as he doesn't enter the token delivery address in the spreadsheet, just copies, it's fine for them.
But in most cases they copy the POA in a hurry, but the address in the spreadsheet is different from the copy of the POA, why don't they in the POA change it and in the spreadsheet they change it.
For that before sending the application check carefully whether it is correct or not, if you don't want to have problems or be accused of an Alt account.