If I'm going to perform a fresh MailWizz install on a new domain, do I need to deactivate my old installation somehow so that my purchase code will be valid for the fresh install?
No, you need a license for each subdomain.Scenario1: Same domain but MailWizz installed on different sub domains and different ip. mailwizz1.example.com (ip1) and mailwizz2.example.com (ip2). Is this permitted?
Again, no. You can only have a single dev environment. for the rest, you need an extra license.Scenario2: Same domain and same ip but multiple installation for experimental purpose only. example.com/mailwizz1 and example.com/mailwizz2
No, leave the license as is in that case, just disable old install after you are done with the new one and you will be fine.
either delete old app or just disable it in backend settings1# How can I disable old install ?
just install/move it to the new domain, then make sure your license info is put in correctly in the backend settings miscellaneous license2# Is that mean, I must install first on a new domain and then disable the old install after that ?