(1) Uninstallers are not always perfectly clean. I guess you are right, though, it wouldn't be dirty enough to leave a major item in Task Scheduler. But check out what Autoruns has to show. Stuff could have been left in the Registry, even just a stray .dll, that could start something up.
(2) I never thought of Superfetch (now called Sysmain)...
https://www.makeuseof.com/tag/what-is-superfetch/
What Is Superfetch (SysMain) on Windows 10? And How to Disable It
I see what you mean. It might preload something based on past usage. But that too should have trouble after a thing has been uninstalled. Still, looks easy enough to disable temporarily for testing purposes.