I had the same issue. Then i switched to the new outlook GUI. the search in old GUI doesn't work. But in the new GUI it works properly. The search is available over Titel Bar.

move tho the new outlook GUI. The search doesn't work on the old GUI but switching to the NEW outlook GUI solved the problem

you need to add them manually. depending on your setting you might have to export your emails first and import afterwards

I use IMAP beside office365, icloud, ..... You can not converter it automatically. but you can re-add the IMAP account after you switched to the new outlook .