1. How to import private key to a wallet that created by seed. Sweep is unsituable if you want to use vanity address. I found the way is just create new separate wallet by importing keys.
Creating a separate wallet is the only way
2. Will addresses from "used" category be deleted ever by Electrum? I don't want to lose them.
No addresses
don't expire and they are never deleted.
Thanks for reply!
And, it is imposible to manage addresses you send coins from, rigth?
This is called coin control and electrum has had this option for a long time now. On the addresses tab select the addresses you want to spend from. On PCs you can select multiple addresses by holding down the ctrl key when clicking on an address. Then right click and choose "Send from". There is no need to freeze addresses.