I'll assist you with your bank transactions, JamieMarshall.
I tried to delete and edit some transactions on my Chart of Accounts and was able to successfully deleted and edited them. I also tried to create a journal entry, use it as a payment, matched and deposited it to other transactions and the same thing happens.
Let's perform some troubleshooting steps to see if this is just a browser issue. Start by using an incognito window and delete it from there. Please also clear the cache to fix errors cause by the junk files on your browser.
I've included this article on how to properly match and categorise bank transactions in QuickBooks Online.
Please post again if you need anything else. Have a great day!