1) My ADA ‘coins’ reside on Binance and I don’t have the private keys. Therefore I don’t really own those coins. They are held on my behalf by Binance. In theory Binance could cease operations due to regulation, corruption, war, whatever and that is the last I could see of my assets.
Correct. There are several other reasons such as phishing, hacks, poor security, new KYC requirements, and so forth that mean keeping your coins on an exchange is universally considered a bad idea.
2) I Install a desktop wallet, let's say Yoroi for Cardano. I understand that i do not create a 'seed' or private key to be held locally by that specific install on my PC, as using the Ledger device, it translates my private key (which is encrypted and held on the device) and communicates with the blockchain via Yoroi.
Also correct. Your private keys are held within the secure element of your Ledger device. The software wallet (in this case Yoroi) is simply an interface between your Ledger device and the relevant coin's blockchain.
Therefore the Yoroi wallet install on my PC is not a great concern, I don’t need to back it up right? I could just re-install Yoroi, or install on another PC and still access my coins using the ledger?
Correct.
Once confirmed I could uninstall Yoroi and forget about it? Six months later, re-install Yoroi on another computer, plug in my Ledger and see all of my transaction history?
Correct.
What happens if development stops on Yoroi? What about if Daedalus offers support for Ledger – could I just install and set that up, plug in my Ledger and see my stash of coins?
Then you could use an old version of Yoroi, or a different piece of software altogether. I don't own any Cardano, but the Ledger website (
https://shop.ledger.com/pages/supported-crypto-assets) says it is also compatible with the "AdaLite" wallet for Cardano.
I think what I am getting at and trying to understand is how important is the wallet itself if secured by Ledger.
Your Ledger device, and the 24 word mnemonic phrase it generated for you when you first initialized it (which you should have backed up by writing it down on paper), are what give you access to your coins. The software you install on your computer is simply an interface. You can uninstall, reinstall, delete, etc., the software, and your private keys are always safely stored within your Ledger device. If you device breaks or is lost or stolen, then your 24 word phrase is your backup to give you access to your private keys again. If you lose both device and phrase, your coins are lost forever.