Ok.. as I understand, I can use my original-seed (created during the installation of my former wallet-app)
in order to be able to restore the fund's into Electrum on Tails (without persistence).
Yes.
Does that mean, all my bitcoin-funds are save(and restorable with the former/inital seed) even the
ongoing transaction is interrupted, e.g. because Tails/Electrum crashes (e.g because of a power outage)?
Your funds are not stored in the wallet. Your private keys to the addresses are what makes them spendable for you. As long as you're able to control your private keys (or your seeds), you are able to control the funds. As for the interruption of outgoing transactions, if it crashes before it gets broadcasted, then you will have to create another transaction again; it'll be like as if the transaction has never been made. If you have already pressed broadcast and it was done successfully, your transaction will show up as per normal.
Are "Transaction-labels" a list of the performed transactions?
Nope. Transaction labels, payment request, address labels, etc are all stored within the wallet file locally, they cannot be restored using the seeds only. It is important to note that while you'll be able to restore your addresses with the seeds alone, you need to have a backup of the wallet file if you are using Lightning Network and have an open channel.