Try unchecking "Store presets with this catalog" under Edit> Preferences> Presets. You can also click on 'Show Lightroom Presets Folder' and the various preset types are stored in the subfolders here. If the presets are missing you will need to download and restore them from Dropbox AND figure out what went wrong!
I would also make backup copies of all image files, catalogs, etc. to a separate backup drive. I wouldn't rely on Dropbox being bullet-proof!
I just could NOT figure out where the old files were stored...by unchecking the box, I was able to see them and export them and then recheck the box and import them.
I agree with you about dropbox, but I have the dropbox files on 3 macs and two external backups to the iMac so I think I'm pretty covered. It's so much nicer not to have to remember to copy a template or setting to the other machines.
Have a great weekend
Sounds like you've got it covered!