1. no, you don't have to uninstall Notion4 if you don't want to. You do need to download the Notion5 sounds to go with Notion5 however.
2. In terms of bundled instruments, no, the instrument list is the same as Notion4 - however there are new add-on instrument packs for Notion5 that are not compatible for Notion4. Plus in terms of 3rd party VST support, we added preset support for LA Scoring Strings and Samplelogic/System Blue marching sounds.
3. With Notion4 you have 2 authorisations, so you should be ok. Notion 5 has 5 authorisations. Contact Tech Support if you get stuck and they can help you out. (Make a ticket via your account at my.presonus.com)