How to manage multiple categories and entry url formats
I can think of a couple possible solutions here:

1. Rethink your url structure
You could always remove the category segment from the url. Maybe create a field for the main category and then another field for all the categories, and use the main category field for your URL. Or you could fetch the first productCategory instead of the last one, so there's less chance of it changing. In my opinion you're going to have a lot of SEO issues if your url changes whenever you add a new productCategory, not to mention broken links and all sorts of other headaches.

2. Do some fancy stuff with routes and entry queries
I think technically you could set up a route like flowers/{productCategory}/{slug} and then in your template do an entry query for an entry with that slug. But probably better to think over #1 and find a cleaner solution.