Those a good questions.
"The system sounds quite like the current one (replace merchants with banks)"
No because currently you only have one currency issuer, one central bank to choose from.
"Why did we move away from merchant-generated and backed credit notes?"
It was impractical. Too many currencies means to many conversions and exchange rates to keep track of. But now we have digital and that can be automated to the point that it feels invisible.
"What/who do you define as a "merchant" in 2024? Amazon? Google? Target? Pick 'n Pay?"
Sure but local merchants that supply you with food, transport and other daily necessities are probably better candidates.
"And since it is 2024, can I send/trade these internationally on a phone with immediate settlement? Can I stream value (worth sub-cents) over the internet with these?"
I think Hawala type chains of intermediaries will emerge over time but don't worry too much about it now. Paying a Youtuber from Manila is an exotic corner case. Worry about your daily necessities so that you don't starve to death because of a financial breakdown.
"That sounds like a lot of "trust" is required again, this time it's the merchants (perhaps re-read the BTC whitepaper, we have moved on from a trust based system)"
Trust is not a bug, it's a feature. Most people are socially apt enough to build trust relationships and those are very valuable because commerce thrives on them. A lot now, even more so in the future. Bitcoiners are smitten by "trust less" because they are socially inept but that's just the few of them. I think life isn't for them and they will perish.
"Could they debase my life savings and generational wealth I want to leave for my grand-kids and beyond?
Could they declare the credit notes they issued are not longer valid/accepted? (I.e do I need to appeal to an authority, as is the case now)"
These things can all happen with one or two merchants but not all of them at once. As for generational wealth, it probably won't exist, your children will have to earn their own living just like you did. Teach them skills how to do so that's the best you can do.
"What are their credit notes backed by and what defines their value? Or are they purely "fiat"?"
The grocer will set prices like 2.5 credit notes will give you 1 cucumber. You will be able to repeatedly redeem notes and from the experience you can extrapolate that this will continue to happen.
Watch "Shillings from Heaven" (on YouTube) for an approximate example.
"What is the path from where we are today to going back to merchant-generated credit notes?"
Merchants can just start issuing them. You can be the first. Maybe make an app.
"Any central authority(s) will be shutdown by govs (see previous attempts to re-introduce billion based money and previous e-cash projects)"
This is a bit of a worry, see the above movie too, that's why we need things like nostr. If nostr is social media that can't be shut down then merchant credit that can't be shut down will work the same way.
"Suggest reading  "Broken Money" book to understand why a merchant-based system wouldn't work, again - especially in 2024 and beyond."
I could do that but nostr:npub1a2cww4kn9wqte4ry70vyfwqyqvpswksna27rtxd8vty6c74era8sdcw83a is an unskilled hick that lives on other people's money and her writings are probably a waste of time.