Electrum offline wallet

[theta]

I downloaded Electrum, I turned off wifi, started a new wallet, copied the seed phrase somewhere safe, and also the public master key, then closed the program.
Then I turned wifi back on, and started electrum again. I selected new/restore and entered the public master key and it opened the wallet in watch-only mode. I noticed that the receive address is the same on both the offline and online wallet.
Am I right in the below?
1) Assuming that my computer was clean when I first created the "offline" wallet (not perfectly offline, hence the quotation marks), it is safe to keep using the online watch-only version for receiving coins and checking the balance, if I don't need to spend the bitcoins in the foreseeable future.
2) Specifically I can send BTC from somewhere else (specifically from coinbase and blockchain.info) to the receiving address there. The coins will arrive safely with no risk of hacking/theft (assuming I don't open the actual "offline" wallet any more - in fact I have removed the actual files from the computer and backed it up) and I will be able to verify it from the watch-only wallet I run.
3) Assuming I always have access to the seed phrase I can have full access to them in the future if I wish to spend them. In that case I either run the "offline" wallet again or start fresh from the seed phrase.
Thanks

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[Wusolini]

I downloaded Electrum, I turned off wifi, started a new wallet, copied the seed phrase somewhere safe, and also the public master key, then closed the program.
Then I turned wifi back on, and started electrum again. I selected new/restore and entered the public master key and it opened the wallet in watch-only mode. I noticed that the receive address is the same on both the offline and online wallet.
Am I right in the below?
1) Assuming that my computer was clean when I first created the "offline" wallet (not perfectly offline, hence the quotation marks), it is safe to keep using the online watch-only version for receiving coins and checking the balance, if I don't need to spend the bitcoins in the foreseeable future.
2) Specifically I can send BTC from somewhere else (specifically from coinbase and blockchain.info) to the receiving address there. The coins will arrive safely with no risk of hacking/theft (assuming I don't open the actual "offline" wallet any more - in fact I have removed the actual files from the computer and backed it up) and I will be able to verify it from the watch-only wallet I run.
3) Assuming I always have access to the seed phrase I can have full access to them in the future if I wish to spend them. In that case I either run the "offline" wallet again or start fresh from the seed phrase.
Thanks

1) Correct. Watch only wallet prevents sending those BTC it's used purely to monitor balance and generate deposit address.
2) Correct.
3) Correct.

Just to add:
When sending BTC you can use the offline wallet to sign the transaction and then broadcast it (the signed transaction) using any other online device. (no need to connect the offline wallet to internet directly)
or
as you wrote above you can restore entire wallet using the seed phrase.

[theta]

Thanks.
So in order to send/spend BTC I will need:
 either the 12 word seed phrase (to recreate the wallet including all its contents from scratch)
 or the actual wallet file (only the small 2kb file inside the wallets folder, right?) and the password, correct?

Another follow up question: If at some point I open the wallet file in electrum and connect to the internet, if my computer is not compromised presumably nothing will actually change in the setup when I disconnect. The offline wallet will have spent some time online but assuming nobody stole it or found the private keys it can still go back to cold storage and be safe from theft, correct?

[Low Sigma]

Hi Theta, Sigma here

Be aware of  keyloggers/malware, they can record/extract information when you are offline and then send the information outside when you will connect back to the internet. Better use a live linux distribution cd(so it is read only) and no internet connection.

[theta]

Hi Sigma 
yes, I understand that. I know there's this risk, I think it's very small as I don't do anything risky on my computer, but that's why I said assuming it's clean at the moment!

[Wusolini]

Thanks.
So in order to send/spend BTC I will need:
 either the 12 word seed phrase (to recreate the wallet including all its contents from scratch)
 or the actual wallet file (only the small 2kb file inside the wallets folder, right?) and the password, correct?
...

Yes, that is correct.

...
Another follow up question: If at some point I open the wallet file in electrum and connect to the internet, if my computer is not compromised presumably nothing will actually change in the setup when I disconnect. The offline wallet will have spent some time online but assuming nobody stole it or found the private keys it can still go back to cold storage and be safe from theft, correct?

Yes but true definition of COLD STORAGE is when the wallet NEVER come online (once came online it's not fully a cold storage).

Sum up:
Anybody who gets access to either:
1-your unencrypted wallet file (encryption must be strong otherwise it's not big deal to break it)
2-your wallet seed phrase
Can spend all your bitcoins ( no matter how safe you stored the other thing !!!)

So the weakest security point is at the moment of creation the wallet so if your PC was compromised that time (and the hacker had gain access to your private keys) he can just wait till you store some BTC there.

But honestly I think this entire offline/cold wallet thing is mostly for people who keep their entire wealth in BTCs and loosing them would ruin their life. IMO using simple home PC with proper antivirus, not visiting doubtful site ... just keeping general online safe habits is enough to be safe.