I'm missing a question:
4. If you are using Google Webfonts, do you use the preferred method by google (loading from Google servers) or hosting them locally?
and probably:
5. When using local copies of Google Fonts, do you observe the font list for availability and if the local files are still up-to-date? (used monospaced Droid font in the past which is no longer available for free)
Until last weeks I've used to use the recommended method and let Google serve client-optimized css declarations, but with upcoming GDPR I've changed my mind and reworked all my sites to host the required fonts locally. Of course I've started using the already mentioned webfont-helper but ended with an own PW module which takes the Google font string (like 'Didact+Gothic:regular|Syncopate:regular|Playfair+Display:italic'), downloads all required variants and creates the required CSS which makes transition rather smooth (if anyone volunteers to make a well documented PW module out of it, please send me a PM). That way I (or my customers) may continue manage required fonts from the Admin.
And, to at least answer one of the original questions:
Since my projects are rather simple with mostly a single CSS file (may be built from LESS) which is included within the <head>, so it was rather simple to use relative paths for now. For more complex sites I'd probably use absolute paths as well.