Retouching is an art to say the least. I have an Adams retouching machine, which helps a lot, but it still isn’t easy. I haven’t used the powder product you mention so I can’t help you there. I would think mixing it like watercolor would work but I don’t really know.
It depends a bit on the film, but you can retouch on either side. It would seam that if you work on the back, you could wash it off if you mess up, but I have never been able to wash it off successfully. I think some of the dye soaks into the film and won’t come out. Anyway, whatever you do, experiment on scrap negatives until you are sure of your technique. You need a very good, very small brush, a bright light box and a good magnifying glass mounted on an adjustable stand or jewelers glasses. With the red dye, you have no way to match the tone so all you can do is turn the spot into a white spot on the print that you can spot to match the print tone later.
Good luck.
Jerome