Am i correct, that your suggestion is to send «unconfirmed» BTC from HD back to MB Classic? It must be one big transaction or two smaller (as i send it to HD)? Where can i read more how this method works?
Sending unconfirmed BTC from HD Back to MB Classic won't do anything but put another unconfirmed transaction in the mempool... he got it backwards. I think he meant to say "send again from MBC to MBHD"... effectively, a double spend. The only problem with this is that you can't easily do that with MBC. It will not let you double spend. So you'll have to do it manually.
Did not know about that fee so important, shame on me. But what if my transaction will never be picked up, because of low fee? Is there any «timeout» for confirmation, when transaction canceled and BTC going back to wallet?
Theoretically, it could stay forever... realistically, a few days assuming no one rebroadcasts your transaction. If you want it to drop... DO NOT open your wallet or MBC until you have seen that the transaction is no longer showing on block explorers like blockchain.info and blockcypher.com etc... If you open your wallet, you run the risk that it will rebroadcast the transaction again!
@HCP, what do you think about «Fix Unconfirmed transactions» option from @davecarter?
Double spending is not easy to pull off... you will need to manually create the transaction and then find some nodes to broadcast your new transaction. Most will refuse it since your first transaction is already in the mempool... also, there is simply no guarantee that the second transaction will be confirmed ahead of the 2nd one...
Could i migrate wallet from Classic to Electrum now, or i need to wait before BTC (somehow) come back?
You can do it now... if you import your keys to Electrum, it should automatically find all the current transactions anyway... It won't necessarily "fix" the current situation, but it won't make it any worse