#2: You’ll have noticeably faster indexing, searching, and folder loading with your photos and sidecar files on a fast SSD. I do all my work on C: which is a 1 TB SSD, then move it over to a slower HDD for archiving when I’m done. For other operations, it doesn’t really matter IMO.
#1: This has been much discussed elsewhere. Presently, PhotoLab relies a lot on its database for a complete record of keywords, projects, and history. WIth PL5, I understand you can export more metadata to sidecars - but the process isn’t complete.