Now, both Joanies are gone.

I wonder how Susan's Joanie would have been written in the main "Happy Days" show?

Certainly different to Erin's

And, to us "Family Affair" fans, Susan was "Lana, my beautiful Lana" to paraphrase the Rob Orbison song.

https://en.wikipedia.org/wiki/Susan_Neher

Three actresses played Joanie Cunningham.

#1 - Susan Neher (Love, American Style segment) - redhead
#2 - Louise Foley ("All the Way" unaired pilot) - blonde
#3 - Erin Moran - black or brunette

Louise Foley is still living according to her IMDb page. She played Audrey Pfeiffer on Family. https://www.imdb.com/name/nm0284188/

There are a couple threads for the unaired pilot.

From Wikipedia (https://en.wikipedia.org/wiki/Happy_Days_(season_1)):

Notes: Two versions of this episode exist. The original version of the episode featured a few alternate takes and extended scenes. Additionally, a different actress portrayed the role of Joanie. The dinner table sequence at the Cunninghams' house was later reshot with Erin Moran in order to accommodate syndicated airings. Also, due to a mistake, Arnold's Drive-In was spelled as "Arthur's Restaurant," with exteriors filmed at a real restaurant with the same name in Burbank.

Threads:

"The Happy Days" Pilot (https://www.sitcomsonline.com/boards/showthread.php?t=310985)

Unaired second pilot on YouTube (https://www.sitcomsonline.com/boards/showthread.php?t=362582)

I don't know if the unaired pilot is available to view online. It apparently is available for viewing at the Paley Center in New York.

https://www.paleycenter.org/collection/item/?item=T:50316

Not only did Susan Neher play Joanie in the Happy Days pilot, she portrayed Bobby Sherman's sister in his short-lived sitcom Getting Together.