If you have a Carbonate solution, it will have a given pH based on the amount of Carbonate. As you add Bicarb, the pH becomes more acidic and the activity of any incorporated developer goes down.
Of course, the amount of solution and agitation also allow the developing agent, if any, to decrease.
In some cases, the developing agent is blocked to prevent spoilage. Base activates it, and less base means less activity due to less unblocking.
These are guesses, not having run the tests myself, but they are based on sound chemistry.
As for the changes on fixing, well, in many cases that depends on how finely divided the silver is and how acidic the fixer is. In this case, I would suggest a neutral or basic fix that is rather dilute. IDK. Just keep us posted.