First of all, thanks for buying Otto Pro. Your license key has been sent to your email. Copy the key → Open Otto's popup → Click on the last icon (outlined icon of otto) on the header → Put the key in the Input field and click Activate.
Well...yes. You buy Pro license once, you can use it forever
No, if you buy a license once you get to keep it forever. You can find the license in the email receipt that was sent to you when you bought it. If you can't find the email, you can email me at firstname.lastname@example.org