Serious question: Your Apple Cash account is tied to your checking account, correct? Is there a built-in 'overdraft' feature included - meaning that if I try to send $50 from my Apple Cash account and the balance is only $25, will the transaction fail or will it pull the extra $25 from my connected account?
If no, then I see the scenario you stated working... but if it does pull the difference from the connected account, the risk/exposure is still there and the process offers no form of additional protection from fraud or error.
That's a good question - it's not directly funded from a checking account. Instead, it'll show that there isn't enough to cover it entirely with Apple Cash and bring up an Apple Pay prompt and give you the choice to use any debit cards loaded in Wallet as the backup funding. These steps will require authentication either with Touch ID or Face ID w/double-click of the side button. There isn't a direct option of "when I don't have any money, pull from this account" and if you had multiple debit cards, any one of them could be selected as the backup.
You can put your routing/account information in as a way to get money out of Apple Cash (it's handled as a direct deposit), but you could always leave that blank and use any money in Apple Cash to either pay an Apple Card, a person, or spend.
Worth a read since they can probably explain it better than I can :
https://support.apple.com/en-us/HT207881
https://support.apple.com/en-us/HT207882