How to prevent a baker's chocolate mixture from resolidifying so that it can be used in beverages? Is it possible to directly melt baker's chocolate (sliced from bar form), mix with sugar, then add an a sort of "inverting" chemical to prevent it for resolidifying? 

I currently work with baker's unsweetened chocolate. I believe this is the product of cacao butter? Not sure.

What I do with the chocolate is emulsify it in water under slow heat whilst stirring rapidly. Typically a 1:2.5 ratio of baker's chocolate to water, then add 1:2.5 ratio of chocolate to brown sugar.

I add this to chocolate drinks, and I've always appreciated its ability to add thickness to the beverage. But if I can make it even thicker, that would be good right?

The bigger problem I suppose is that I've never trained with a chocolatier. I don't know what the pros do!
I'm not sure if this is what you're looking for, but what came to my mind was making a ganache - pouring heated cream over chopped chocolate and then stirring until the chocolate is melted.  By adjusting the ratios of cream to chocolate, you can make a lovely thick chocolate syrup that works well for stirring into beverages.

There are 4 standard ratios of chocolate and cream, for different purposes:

2:1 (chocolate:cream) is very firm and holds its shape, good for uncoated truffles.

3:2 is firm but slightly softer; good for coated chocolates and cake icings.

1:1 is soft and smooth; good for tart and cake fillings, some cake icings, and spreads

1:2 is pourable and sauce-like - the obvious choice for sauces.



(from a Chowhound article on ganache)