The expiry time doesn't matter at all.
Take note that addresses do not expire and you can still receive bitcoin even after the expiry time. The expiry time is for the request, not for the address.

Whenever you receive bitcoin (even after the expiry time), you will see the new coin in your wallet.

You can't sweep you private key into electrum, if it's offline.
If OP wants to make the transaction using electrum without the private key connecting to the internet, he/she should create a watch-only wallet for making the unsigned transaction and another wallet with importing the private key on an offline device for signing the transaction.

Download an open-source trustworthy wallet like electrum or sparrow, run it on an air-gapped device and generate a wallet.
In this way, you can have a large number of addresses all generated from a seed phrase. You will also have a master public key which can be used for generating your addresses without any need to your seed phrase.
Write down your seed phrase on a piece of paper. (It's recommend to keep multiple copies of your seed phrase in different places.)


This would be much better than a wallet with single private key and address.
Writing down a seed phrase is much more easier than writing down a private key and you won't have to generate a new wallet every time you need a new address.

Your transaction can be flagged as RBF only when you make that and there is no way to turn a non-RBF transaction into a RBF-enabled transaction.
In the case you have made a non-RBF transaction, it will be still non-RBF even if you import your wallet into a software supporting RBF.

But the 100 transactions don't have to be in a single chain.

I can make a transaction with 99 outputs and then spend those 99 outputs in separate transactions.
If I replace the transaction including 99 outputs with RBF, nodes would remove 100 transactions from their mempool.

If by million replacements, you mean replacing a transaction million times, yes. There is no limit for that.
The limit we have is that the total number of transactions that should be removed from the mempool after replacement must not be bigger than 100.

Nodes don't have to keep transactions in their mempool. Each node can have its own settings. There are many nodes that use default settings and drop unconfirmed transactions from their mempool after 14 days.

That's how it has been designed.
I think if you don't open your wallet, mycelium will no longer rebroadcast your transaction.

Yes.


With just connecting your internet connection when signing the transaction, you don't really increase your security. For more security, you should import your private key into a wallet on an air-gapped device.


You should have two wallets. One created with your private key on offline device and a watch-only wallet on an online device.

Here is a good guide.
How to spend from an offline paper wallet using Electrum

I have never used the website in question and I would avoid it even if it was working.
You should never use online websites for generating a bitcoin address.

If you want to have a bech32 bitcoin address, install a trustworthy open source wallet like electrum or Sparrow and create a wallet. In this way, you can have multiple addresses all generated from a seed phrase.
For more security, It's recommended to generate your wallet offline on an air-gapped device.

If you are on the desktop version, click on the circle located at bottom right corner of the window. In the new window, go to "Proxy" tab.
If you are on the mobile version, tap on the circle located at top right corner of the screen. Go to "Network" and after that tap on "Proxy settings".

You don't need to use a wallet that supports bitcoin along with the forked coins.
Import your private key into a wallet that supports bitcoin and move your bitcoin. After that, you can import your private key into a wallet that supports the forked coins to spend them. 


Here are some tips for ensuring the security of your fund.

• Use a trustworthy open source wallet.
  
• It's recommended to import your private key into an air-gapped device and sign your transaction offline.
  
• First move your bitcoin and then go for forked coins.
  

The fee you pay for sending bitcoin from an uncompressed address is slightly smaller than the fee you pay for sending bitcoin from a compressed address.


First note that when we say "uncompressed address", we mean that the address has been derived from an uncompressed public key and when we say "compressed address", we mean that the address has been derived from a compressed public key.

You can generate different legacy addresses using each of compressed and uncompressed formats of your public key. They are both standard and you can use each of them. It's recommended to use the compressed address.

When it comes to bech32 addresses, you can still generate different addresses using each of compressed and uncompressed formats of your public key, but the one that is generated from the uncompressed public key would be non-standard and it would be very difficult to spend fund from that.

You are wrong.
You can send bitcoin from any type of address to any type of address you want. This means that you can send bitcoin from segwit address to legacy address and vice versa.

We have compressed and uncompressed addresses, because the public key can be either compressed or uncompressed.
When bitcoin addresses were introduced for the first time, they were all derived from uncompressed public keys. In 2012, it was decided to generate addresses from compressed public keys to decrease the transaction size.

An uncompressed public key includes 130 characters, while a compressed public key includes 66 characters and takes less space.

If the address is yours, you can know that with checking the public key or the private key.
If there's outgoing transaction from the address, you can know whether the address is compressed or uncompressed with checking the raw transaction.

If the address is not yours and there's no outgoing transaction from the address, there is no way to know whether it's compressed or uncompressed.

Tap on "Receive" button and "Create request". Select "Address" and you will see a QR code which contains your address.
You can tap on the "QR code" to see the address itself.

Take note that every time you do this, electrum gives you a new address and they all are yours.
To see all your addresses in one place, tap on your wallet name at top left corner of the screen and go to "Address/Coins".

With using TOR connection, you can hide your IP address, but you would still rely on a third party for getting transactions data and broadcasting transactions. The server you are connected to can link all your generated addresses with each other.


Your security depends on the security of the your device, how you generated your wallet and how you keep you keys. When it comes to security, it doesn't matter whether you reuse addresses or not.

Coins tab displays your coins and the address associated with each of them. The coins you own are associated with two of your addresses and all addresses are empty. That's why you see two address in Coins tab.
Once a new address of your wallet receives fund, you will see the third address in coins tab.

For making a transaction offline, first you create an unsigned transaction on your watch-only wallet. Then you export the unsigned transaction and import it into your offline wallet. You sign the transaction on the offline wallet and after that you import the signed transaction to the online device and broadcast it to the network.

A paper wallet should be generated offline and the private key should never connect to the internet. Otherwise, you would defeat the purpose of a paper wallet.
If you generated your paper wallet using on an online device, that's not really a paper wallet.

You can import your private key into a hot wallet and spend your fund, but that's not recommended. For more security, it's recommended to import your private key on an offline device (preferably to be air-gapped) and sign your transaction there.
Take note that for making a transaction offline, you would need two wallets. You would need to create a wallet with your private key on an offline device and a watch-only wallet on an online device.
The offline wallet is used for signing the transaction. The online wallet is used for creating the unsigned transaction and broadcasting the signed transaction.


I edited the post to fix a typo.
Thanks Roberto12345 for the correction.

I mean the address which has been generated by your private key in electrum. Is that correct?

As I already said, to check the address generated by your private key, you can tap on your wallet name at top left corner of the screen and go "Addresses/Coins".