I hold ctrl key, but nothing happened. What is LMB?
Left mouse button. Hold control while clicking it.
How to use Master Public Key to track all the addresses?
You can find the MPK in "Wallet -> Information". I used
Sentinel on my Android phone, but you can also paste it
here. Keep in mind that you should be careful with sharing your MPK since it exposes all of your addresses and consequently their transaction history.
When I looked at the blockchain explorer, 18mbtc amount is both in confirmed received and spent. However Electrum add that amount to the balance history. Why?
Aren't you looking at the address of your closed channel? It had to be spent to one of your addresses so that the channel could be closed. Still, I am not sure why there is a discrepancy between the balance in the "History" tab and your total balance in the bottom-left corner.