Interacting with those protocols will require good amount of $ to be spent, and then again it doesn't guarantee anything. Best suggestion I can give is interact with dapps/protocols where you feel like doing so and leave rest.
Right, specially test net airdrop will ask to deposit in their platform, it can be 30$-50$ even more i have seen a few airdrops but there are no guarantee you will get any rewards it’s definitely depend on your luck i think. So, it’s pretty difficult to predict that which project will pay reward or not. Personally i tried with several project and get only from only a project.
You obviously don't quite understand what you're writing about. If you are asked to deposit real money for testing in the test network, then it means that you have encountered scammers and you should be as far away from such a project as possible. As for the message that you quoted, there we are talking about the commissions that will need to be spent for interaction in the main network.
true in testing blockchain usually it doesn't require money to be deposited but instead we can get from faucets, meanwhile the money spent in the main blockchain that is not test blockchain was used for fee.
usually there's nothing charging the testers.