Do to dry out the marshmallows

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.

It's exactly your deviation which caused the problem. Stiff meringue weeps - the foam slowly loses the egg white moisture. When you mix cornstarch in it, the cornstarch absorbs the water droplets before they have the chance to make a big wet spot. So if you use the marshmallow mix, you won't have this problem.

As for crystallization, I am not sure that xanthan gum helps. It prevents crystallization in ice cream, but there you have too much water between the dispersed particles of the other ingredients, and this water forms ice crystals. Xanthan thickens the water and makes the water droplets smaller, so they don't freeze to big, crunchy crystals. In a sugar syrup, you have it the other way round. Sugar is better soluble in hot than in cold water, so once the syrup cools down, there is too little water and too much sugar. I don't think that xanthan is capable of doing for the sugar what it does for the water. But even if it helps, it will make your result firmer than you want it, you'd have to reduce the gelatin, risking to end up with marshmallows either too firm or too soft.

The usual way to prevent sugar syrup from crystallization is to add an acid to it. This splits the sucrose molecules into glucose and fructose, which don't crystallize at the saturation sugar does. Be careful what you add, you don't want your marshmallows to smell of vinegar. Cream of tartar is the neutral smelling choice, and citric acid has a smell, but it doesn't feel out of place in most sweets.