Electrum don't allow importing private keys (only sweep) for this specific reason because people then won't be able to restore that imported private keys using the seed as It wasn't generated from it.
Yup for those wallets that were generated with a seed.
So every seed has limited number of available addresses?
No. I don't exactly know the limit but each seed should be able to generate unlimited amount of addresses, I've never seen anyone going into a situation where they can't generate more addresses.
That would mean, that every seed has also limited number of transactions - since rest of the funds is always moved to a new address.
No. The funds are always moved to a new address because of privacy. Imagine
My transaction is:
Address A -> Address B
-> Address A,
You would know how much Bitcoins and who did I want to pay.
However, if another transaction is:
Address A -> Address B
-> Address C,
You wouldn't know if Address B or C is mine.
