Well it "can" be returned after 14 days, but a "malicious" node can also just rebroadcast it to prevent that from happening.

Bitcoin transaction are never "returned" - they are simply either confirmed or not yet confirmed.

Technically they can be 'returned' by using a higher fee to move the coins to a different address making the original transaction un-confirmable.

Unless "replace by fee" is opted into, most nodes won't allow a second transaction to replace a previous one.

Which means it's a gamble if that will ever work.

If the transaction hasn't been committed (etched into the blockchain), then isn't there a financial advantage to accepting the highest fee transaction?

The protocol doesn't matter, mempools aren't validated or shared[+], and so the validity of the chain isn't challenged by processing the higher fee.

So given that the bitcoin network is designed to be operated by greedy miners, surely one would break with "tradition" and process the second?

You might have to dig to find a route to those nodes but it should surely be possible.

[+] They're not shared as in shared-state, i.e. sync'ed.

Yeah, which is why many will "break ranks" and include a higher transaction fee even if there is a previous spend in the mempool.

But it's playing with fire to rely on that, because without opt-in RBF it's far from a sure thing.

the mempool ignores transactions older than 2 weeks: https://github.com/bitcoin/bitcoin/commit/5f0e27f1a8495d9be4...