Depends on context. When the transactions are unconfirmed, merchants are more inclined to accept those transactions without optin replace-by-fee enabled when they have no confirmations since it can easily be replaced. When they have one confirmations, both are pretty much equally safe.
I would say its more of Electrum giving the user an option to choose what they want and what they don't. As stated, opt-in RBF gives the user the freedom to replace the transaction with another transaction with higher fee when it is unconfirmed. There are many useful situations where you can use this feature, etc low fee and wrong address. Merchants would never accept opt-in RBF transactions with 0 confirmations.

think of RBF as a flag you put on your transactions (any kind of transaction from any address to any address). and that flag is only telling anyone looking at the transaction that this transaction can bump fees if it was needed.

in technical terms you just change the sequence number to a lower value and each time you "bump fees" that sequence value is increased to a higher value. Electrum uses 0 as its initial tx with RBF.

You can. What you are referring to is FSS (First Seen Safe) RBF where the miner will assume the first transaction that has seen the input as the reference transaction and the outputs in subsequent transactions has to have at least that value to the specified address.

The answer is that is the same. They are essentially the same thing, just a little change in the sequence which basically signals for opt in RBF.

There is no difference in security between them.

Yes.