At the start of a run, say with 0.1 m/l of NaBr at 40 deg C, the pAg will be about 10.5, the vAg will be about -50 mv, and the pBr will be about 1.1 So, in a single run, the pAg is high at the start, not low. It decreases as you add silver, so that if you add 0.05 m of silver to this kettle, the pAg will drop to about 10.2, the pBr will rise to about 1.4 and the vAg will be about -34 mv.

So, you have it pretty much backwards in your first paragraph.

A double run has two effects. It makes the size of the crystal more uniform, and it controls the type of crystal formed. A chart of the latter was posted here in another thread.