My son used a pressure washer to clean the cedar fencing around my house, and the dark discoloration on the fence was a combination of mold/ mildew. He didn't put anything in the water since the pressure was strong enough to remove it. I"m thinking it would be the same on brick.

Likely pavement ants. A seasonal nuisance.

Carefully loosening the sand in the joints and shop vac'ing out. Time consuming but should be damage free.

As long as your installer used a deep enough base of stone, and used a fine packing stone for the top layer you should be good. At my company, we use what's called a "dirty stone" which is a mixture of course and fine gravel. We put down a level of stone and compact it. Then we put down another level and compact it again. We repeat this several times to insure the paver stones will not shift and the ground stays level. We like to use a thin layer of sand as well, then compact again. After installing the pavers, we compact one last time to help lock the blocks in place. As long as the stone is compacted you shouldn't have any problems. Here is a link that might be useful: Paver Stone Driveway and Patio Installation Tips in New Jersey