Larry Dighera wrote:
Are ultralights only subject to FAA Part 103, or must ultralight
pilots meet Part 91 regulations also?

Yes and no, respectively.

So, if it was a Part 103 ultralight, Part 91 is not applicable
rendering the sheriff's mention of remaining 500' from persons and
structures incorrect, no?

Somewhat, but they can still stick the pilot.
Sec. 103.9 Hazardous operations.

(a) No person may operate any ultralight vehicle in a manner that
creates a
hazard to other persons or property.-c