As for the first problem a slightly loose freehub this happen's but not often usually the result of it not being fully tightened and is usually fine if you make sure the bud is correctly seated and just retighten.

As for the brakes I can say from experience you should be able to get acceptable to very good braking out of that setup. Another thing to check since your running MTB pads make sure the washer stack is correct which alloys you to align the pads with the rim with slight tow in the brake goes between the 2 dish washers.

I have a very similar brake setup on my current daily rider and the brakes are very good better than most of the calber brakes I have used.