After how many hours a transaction was made without receiving a confirmation, it's recommended to intervent?
Do you want to get it confirmed soon? If yes, then you should intervene.
Is it possible to define a new output with a higher transaction fee (in Electrum on Tails)?
Yes, you can “double spend” the amount you broadcasted with a higher fee if the transaction was signed as RBF. In electrum, it should allow you to do that, because it chooses RBF by default.
It should not differ on Tails.