Thx folks!
So c) seems possible - importing private keys. But what does it mean? Do I then "own" the private keys in the new wallet or do I still have to transfer the funds to keys of the Electrum wallet? I understand Electrum uses deterministic wallets, I.e. can those imported private keys be recovered from the wallet seed? If no, what's actually the difference between b) "sweep" and c) import?
The more I look into this, the more complicated it seems to get
Sweeping means that the amount will be sent into one of the address generated by the seed. Importing means that the private keys is imported into the wallet. This means that in sweeping mode, you have to only remember the seed but when you import, you need to remember both the seed and the private key if you use the addresses generated by the seed.
IIRC, Electrum sends back the change into the input address if it is imported while it uses a new change address for seed addresses. Since you could use an address generated by the seed and imported address at the same time, the change would be sent to either of them. Correct me if I'm wrong.
Still, I don't recommend you to delete your private key in either of the mode.