These "double spend" and "low fee" warnings are causing me concern about the idea of trying to send the second transaction again. I don't want my money to be stuck in some kind of void of never completed transactions. As I didn't get to see the transaction fee, what if this was Zero!?
There is no such void, transactions go to the mempool when broadcasted before it is added to a block. It would remain here for approximately two weeks before it is dropped, if not picked up for confirmation, after which it returns to your wallet, although it technically never left.
So long as the address you entered initially was correct, you can try again and ensure you click on pay to sign the transaction.
Toward the bottom of the main post there is a small Electrum section. The last para says:
To increase the fee of a transaction that uses Opt-In RBF, right click the transaction in the history list and choose the "Increase Fee" option.
... This is where I am certain that I can't see the second transaction, not in History anyway. I can see what looks like the first one though.
If it's not in your history, then it wasn't sent. You can confirm by checking your address on a block explorer (like blockchair) to see all recent transactions. Electrum allows you to bump transactions after broadcasting, if the fee set turned out to be too low, but you need to have set the bump option before signing the transaction.
