Why the balance is different between BRD fast sync and electrum ? (I read somewhere that multibit HD had very low low fees )
Exactly 100 times less? Did you check the denomination used in Electrum?
Is there any risk to loose BTC during sweeping process because of these multibit low fees?
No. The most that could happen is that the transaction gets stuck and/or the transaction cannot get broadcasted.
I am little bit lost. To adjust fees in electrum, it seems i need password keys but they are encrypted in aes.
Did you use the seed and import into Electrum?