Is it safe to roast marshmallows over a sterno flame

The easy way: Use raspberry oil (preferably) or a raspberry extract in place of the peppermint oil.

The (probably) much better way: Leave out the oil and food coloring and use raspberry puree as you suggested. Strain frozen or fresh raspberries through a fine strainer or cheese cloth. Weigh the resulting juice/puree and then put it on the stove an cook it until the puree is half of its original weight (roughly). This reduced puree will be more intensely raspberry, so it will get all of the raspberry flavor without having to add as much water to the recipe. You'll have to experiment with the amount of reduced raspberry puree to use.

I wouldn't increase the corn syrup, but you certainly could if you want them sweeter. I would personally go for the opposite effect and add a small amount of lemon juice to the raspberry puree to give a little tartness to complement the sweetness of the marshmallow.

As far as the chocolate, I would recommend a darker chocolate. 55% is quite sweet, especially around a marshmallow. You should also consider the type of chocolate. Cocoa percentage isn't everything in dark chocolate. Experiment with different types of chocolate and see what fits well with the marshmallows. Just like wine, chocolate should be paired with other foods according to the subtleties of the taste of the individual chocolate.

The vast majority of recipes I find call for marshmallows to be added at the last second, and cooked only long enough for them to melt. Usually then a heavy frosting is added to top the whole thing, giving the illusion of marshmallow inside the brownie. Find an example here.

I've also seen really "wet" brownies with lots of included chocolate chips and butterscotch, which contain non-disgusting looking marshmallows. I've never personally tried these because they seem overly sweet, but they're the only ones I've seen that contain marshmallows that seem to retain elasticity after a normal baking period. See examples here and here.

For my money, I'd probably just sprinkle some marshmallows over the top of regular brownies during the last 3-4 minutes of baking. Quick, simple, not overcooked.