Hi VivF
The answer to #1 depends on whether you bought a full (standalone) version of PD or if you paid for an upgrade version. That's the least expensive option, and that also means you're not allowed to use PD13 and PD12. If you paid for a full version, you can keep both. if you want to.
With PD13, you can access all the projects and any content you may have downloaded from PD12, even if you uninstall the older program.
If you keep both versions, please know up-front that once you create or save a project in PD13, you cannot open it in PD12 after that. New programs can open old projects, but not the other way around.
For #2, you should definitely install the PD13 v3130 patch because it fixes several issues. As for the AVI fix for Win10, you can go with "wait and see" because only certain clips are affected. If you ever encounter a "Media Error" message with any clips on your timeline, that would be a good time to follow the steps listed in that sticky.
Happy editing
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