I suspect you can import it directly from the console within Electrum (based on this PR). Currently, Electrum does now allow importing individual p2wpk inside p2sh addresses (the 3 segwit addresses). You can, however, still import an entire seed by using a BIP49 derivation path.
Try running the following in the electrum console:
key = 'your_key'
Calling importaddress will rescan the whole blockchain in default according to bitcoin.org
Set rescan parameter to false to not rescan the block database
(rescanning can be performed at any time by restarting Bitcoin Core
with the -rescan command-line argument). Rescanning may take several
importaddress "address" (...
It is not so easy to "open" a wallet, because it requires rescanning the blockchain to find the outputs that are relevant. This can take a very lock time, especially if you do not have an SSD. Because the performance is so poor, it would be disappointing for a lot of users to use this hypothetical feature.
Form bitcoin-abc I have found this
# Enable pruning to reduce storage requirements by deleting old blocks.
# This mode is incompatible with -txindex and -rescan.
# 0 = default (no pruning).
# 1 = allows manual pruning via RPC.
# >=550 = target to stay under in MiB.
So, the command import address should call the -rescan (in Bitcoin core yes),...
You're using on incorrect bitcoin-cli method to list your addresses.
The command you're looking for is:
bitcoin-cli listreceivedbyaddress 0 true
The 0 parameter means you're requesting information on any address and the true flag means you want empty addresses included.
The original command you were attempting, listaddressgroupings, is described here.
You need the private key not the public key to do a sweep.
Actually it'll be easier if you simply copied the wallet file to the new system. If you have access to the electrum gui then you can create a copy via file > save copy. If you don't then find the wallet file and copy it over.
A wallet is not related to whether the blockchain has to be downloaded. The blockchain is not stored in a wallet. Rescanning does not redownload the blockchain unless you are using a pruned node.
To use importaddress, you must have a wallet. It cannot work without a wallet file. If you don't have a wallet file for some reason, starting Bitcoin Core normally ...
Is there any way, other than importaddress, to watch balances of specific addresses?
No, creating a watch-only address is the way to do this using a wallet such as Bitcoin-core.
how it is possible for wallets and explorers to give information ... even though importaddress takes some time?
Once the blockchain has been initially scanned for transactions ...