(1) Have set Em Client to be my default, but it (Em Client) insists it is NOT my default. Everything works anyway, so no biggie, just annoying.
See this thread for help on setting eM Client as default in Windows 11
Regarding #2, “invisible emails,” upon further fiddling, found the emails in the “Other” folder. I hid that folder, will see if that fixes the problem. Would prefer all incoming goes to Primary folder.
That means that you chose the default recommended installation which enables Inbox Categories by default as per @gary post below and can change that via the Settings.