I a little bit disappointed about it. It's almost 2018' ending, but still no any human-like merchant script that can cover all that problems? Is there really no simple logic to clean up own addresses for e.g.?
What kind of problems ? This isn't a problem at all.
That's a design-decision. It is a trade-off between convenience and efficiency. There is no reason to check thousands of addresses if that's unnecessary for 95% of all electrum user. That's just a slow down.
If one has created 1000 addresses which received transactions, he would simply need to generate 1000 addresses again after recovering to see the correct balance.
Private-/public- keypairs are generated successively, starting with the index
0. There is no risk in losing coins or anything.
Ok, I talking about the way to receive bitcoins from users with ability to notice server about payment through callback function. This simple scheme we can see into any big exchange services, like bitfinex and so on. Ppl just go to their deposit tab, see their deposit address and they can send bitcoins to it, at any time. And any other can do that. I'm not sure, that such services working this way. So maybe Electrum not suitable tool for developing it or I'm wrong?
electrum tends to slow down after a few thousand addresses so you'll have to rotate wallets every so often. also use addrequest instead with the --force switch. see `electrum help addrequest` . you can specify any amount to addrequest. it doesn't have any effect on how much people can send you.
It's kinda deceision, but as for me, it's not cool. As I described, the purpose is give access to any user ability to deposit their
own address.
)
... (Or at the very least, a “hello” from all the angry emailers who want to know why they didn’t get something from your site). 
)...