Since pH is stable, I wouldn't move the TA. I would, however, move the pH up a little with Borax, 7.2 is as low as you want to go leaving little room for it to drop. We've had alot of rain recently and my pH went from the high side of 7.4 to under 7.2 in one storm ( my TA dropped as well ). The rain consumed my margin and I was still in the safe zone - had I started at 7.2... You do have room with your TA so if you want to use Soda Ash ('cause you already have it?) then, sure, just do it a little at a time ( 1/4 cup to start ) - let it mix throughly and measure the result.

The Brown Stuff: Does it really bother you? Is it staining your pool?
If it is metals, you might get more when you raise the pH - watch for that. Is there more when the FC is higher?
Perhaps your observation in the CH test is good reason to have your water tested for metals.