The bank may let you exceed your available balance but most are very careful about doing that these days due to financial uncertainties.
Only if they can hansomely profit from it. That is, they are quite happy to let you overcharge, say, $20, if they can charge you $35 for the overage.
1] Use credit, not debit, cards for internet/airline/etc. transactions
Wouldn't have changed a thing, as relevant to this incident. Whichever card I used, that card would have been blocked from making a 2nd transaction for the same amount, assuming they both had the same initial limits. However, for other concerns, CC might be a better choice.
Don't assume that because the transaction "completed" on the website of the airline you are purchasing the ticket from that everything is hunky dory.
That used to be the case. It's amazing that as we get more and more technically advanced we become, our service actually declines.