Pepsi can probably afford to run Coca Cola through a mass spec to get an idea of concentrations and even get the processed coca leaf used by Coca Cola (there’s one company in the uS with a license from the DEA).
Some people perhaps. Restaurants usually only carry one or the other so you don't get a choice.
With millions of dollars tied up in just a few percent of sales you can bet Pepsi knows just about as much as Coke does about Coke's ingredients (and vice versa of course).
The research for both companies is more about the fine minutiae—keeping an optimal differentiation between the two products more than treading on each other's territory. Trampling over each other for market share is done through advertising, not by making their products the same.
Most trade secrets aren't really all that secret.