Regarding your issues with ISBNs in your personal library, I suspect you must either have had an issue with your lookups/app, or you had several books from a (almost certainly tiny/amateur) publisher who improperly reused ISBNs. I've spent some time working at a bookstore with 80,000+ different ISBNs and I can count the number of issues with ISBN re-use we encountered on one hand.

We'd put pricing barcodes on every book in the store, and those were always based on the ISBNs and had the titles and authors printed on them, which was info that came from ISBN lookups either from Bowker's Books-In-Print data or Ingram's data. We'd print the barcodes in large batches and then have to match them to the books based on the title and author shown and verify with the ISBN, so all 80,000+ were checked, and the actual ISBN issues were _extremely_ rare and always from a _very_ small/amateur publisher.