That doesn't sound like an observership, technically, because according to your description you are hands-on with patients. Observerships are hands-off.
Not sure what you mean by "an internship in a private practice", but internship/residency is almost always matched into through ERAS. You can complete a Family Medicine internship/residency, but it needs to be under the auspices of a residency program and not under a private practice. There are some residency programs that do not go through ERAS, but they are still residency programs and not freestanding private practices.
Definitely mention your USCE specifically. There was someone here on VMD last year who did not mention their USCE, thinking that it would be obvious to the PDs, and they ended up with no interviews at all. As I recall, they had decent USMLE scores and were otherwise competitive, too. Make no assumptions. Mention everything. Be clear. Get good letters. Apply very broadly. This process isn't getting any easier.
"When I havenīt any blue... I use red."