Quote from: NatalyDNA on Today at 01:05:30 AM

In the case i did the choosing BIP39, tried the "Detect Existing Accounts" option, after that Electrum Wallet detected BIP44 Legacy Account 0 providing the evidence for transactions made under Legacy Tab in the Blockchain Wallet, transactions' amounts and dates were correct, would it still be the issue "to sync the "blockchain headers" with the servers before it can display the correct balances" under the Segwit Tab?
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In worst case, you'll only need to keep the wallet open for an hour.
But for normal instance, it will only take a few minutes to complete, after that, balance will sync in seconds.

You can tell that you now got the blockchain headers if any one of your Electrum wallet displays the correct balance, so in your case, it's now downloaded.

Quote from: NatalyDNA on Today at 01:05:30 AM

"You can also try that to check if you can reproduce those addresses into your blockchain(dot)com's unused addresses list." Could You, please, send the hint for this to be managed?

Just re-do the "Add Next Address" that you did last time and check if it will re-create those missing addresses.
I see that you've already did that (based on your quoted post below).

Quote from: NatalyDNA

If i go to "Add Next Address" under the Segwit Tab i can create the new address and it is shown under the "Unused Addresses".

Does the new addresses the same as the "missing addresses"?
If so, it's just blockchain.com's usual issue where they can't display the correct balance.
If that's the case, try to send a small amount to the unused addresses before those newly created wallet,
but if all of the addresses in the "Unused Addresses" list have a balance already, or even if no, start contacting their support.