Congratulations on getting your new k-2006 - it'll make life with your pool much better. One note: there's no need to use the 25ml sample for chlorine testing, the 10ml sample is fine and your reagents will last longer.

Your FC of 4.0ppm is too low for your CYA of 90ppm (hence you have algae). Per Ben's Best Guess Chart your normal FC is between 5ppm and 10ppm. Never let it go below 5 and I'd try to stay on the high side of the normal range.

One metal missing from the list above is iron. Brown stains may be Iron, may be something else.
Did you do the bucket test on pool water and hose water or just on hose water?

Your bucket (hose water) test showed no brown staining.
Your Vitamin C test brought a strange black/purple temporary stain but did not clear the brown stain.
The brown stain was somewhat lifted by a chlorine tab (trichlor, I presume).

Maybe you don't have a metal issue (sorry to disagree with you Ben).

Maybe the brown stains are algae (or something), and so is the stuff in your main drain.

If you didn't do a bucket test on your pool water, I'd recommend you do one now. If the pool water bucket test shows no staining, then I'd recommend you Shock the pool.

How have you been chlorinating? shocking?