2: As wallet id, I meant ''address''. For receiving case, you told me I need to share the address to which I want to receive bitcoins. But what if I want to receive the bitcoins funds to address I have never used before? For this I might need to create new address BUT you told me I cannot manually create address (neither one nor more) within electrum. However you said I always have 5 unused ones. This word ''always'' got my attention. Since you were saying so, I assume as soon as I receive ANY bitcoins amount to ANY address, NEW unused address (never been there yet in my electrum) will Automatically occur. Is this correct or not?
Electrum tries to ensure that you have unused address equal to your gap limit. The gap limit is 5 by default. It means if you receive bitcoins to an unused address then electrum will generate a new one to ensure that you have 5 unused ones. If you receive bitcoins to an address that has previously received bitcoins then obviously there is no need to generate new addresses. They were already generated when you received bitcoins to that address the first time. I hope you understand.
Electrum monitors transactions and creates new addresses like I described above. That is all that happens.
Yes it will but addresses that you have spent from are tucked away under 'receive' tab >'main account' > 'used' to reduce clutter. In case you are wondering you can still continue to use them to receive or send bitcoins in future.