What are those resin filled holes on outside walls
I suspect that the material is a lot closer to glass than resin, and is part of the "look" of this particular style of brick - that is, it came out of the brick kiln that way. 

It might or might not be manganese. One of the potters I know is prone to using a light clay with manganese, which does get brown specks, but they are smaller - however, that's "pottery clay" not "brick clay" and perhaps they are smaller due to using smaller bits of manganese. Otherwise very similar in "look."

Closest I've found in a quick look is "light ironspot" - still looking for a decent picture. Name would suggest iron ore rather than manganese for a material.

In short, nothing to worry about.