There was also fake Sam Franklin. Played throughout the original series by Allan Melvin, the character was played by Lewis Arquette in A Very Brady Christmas.
In the first 2 intros, Carol tilts her head to the left when she looks down at Mike; in the last 2, she tilts her head to the right. It always bothered me how she didn't do it the same way every time.
That was indeed Cindy/Susan Olsen and I totally agree with you on her resembling Family Ties Jennifer, though Tina Yothers herself ditched her blonde look after series ended.