Hi Arthur,
Yes, if you purchase a redistributor license, the on prem InfoPath filler forms injected with qRules will continue to work perpetually. Once a form is injected with qRules, it never expires. It only expires if it was injected with the trial, but this is not true for FormsViewer.
If you are using FormsViewer, the publish and preview from the browser is very fast, so you should not have to use the InfoPath Designer preview and I don't recommend using it because you want to see how it looks in the browser anyway. So, once the form is injected with the trial, it should never expire if you are only using FormsViewer.
In the future, we will remove the qRules injector expiration (if it exists) so that FormsViewer customers won't have to worry about reinstalling a new trial to inject a new form. :)
I hope that answers your question. We look forward to supporting you and your clients.