Woulda, coulda shoulda, but I imagine that if A. left rather than Marcy there was Robert Barr to take up the reins, except that the Dobsons didn't create that character so they were not motivated to write for him.
She had a good contract with NBC for other primetime movies (according to IMDB after Palace Guard she starred in four NBC movies), so I think it was simply money. I recall when she was later hired at GL, there was press about how much she was paid, and one assumes that SB did not have the budget to make her an offer she couldn't refuse. Or, given all we now know about the mismanagement at SB, perhaps there wasn't enough money to make her want to go back.
Just discovered that actresses Jimsey Somers and Jocelyn Somers are in fact the same person.
Jimsey changed her name to Jocelyn as she matured and wanted to not be associated with 'child actress' tag.
Edge of Night March 29/30 1961