Electrum does not support bitcoin core wallet files. What you can do is dump all the private keys using bitcoin core and then import them into electrum. So first make sure that the wallet.dat file is in bitcoin core's data directory and then launch bitcoin qt, go to window menu > console and unlock your wallet:
walletpassphrase your-wallet-passphrase 300
1) How can you run a full node wallet with electrum? what do you need? Or please link me to an understandable guide for that
This probably should be a separate question.
2) Do you have to be online always or can you set it up, fund your wallet, and leave it offline for months, and just from time to time use it for transactions?
You can use it only ...
What other full node wallet apps are available to select from for bitcoin?
See https://bitcoin.org/en/choose-your-wallet for a reasonable list
Is it mandatory to have an own server when using a full node wallet
Not really. Some wallets have an API that allows other software to interact with the wallet but the wallets are generally usable directly for ...
Follow this guide.
Download Electrum and verify the signature to prove it was not modified in transit. Create a new Standard Wallet and choose "Create a new seed". Electrum will give you a seed phrase which you should write down on paper.
If using Bitcoin-Qt, click Window -> Console.
If your wallet is encrypted, use this command to decrypt it for ...
A single private key can lead to different addresses, depending on the script it is used in - These include p2pkh, p2wpkh, p2sh-p2wpkh.
Electrum prepends the script type to the private key during export and import so that it knows which address to derive and check for outputs on.
Well "wallet" as storage of keys should always be under your control. There are online wallets for higher convenience, but as the old saying goes "not your keys, not your bitcoin". It's probably a bad term, but I'd rather say "local (SPV) node" instead of "wallet".
When you are sending there is no problem with malicious peers as the transaction is (usually) ...