Regarding the leaf damage on your plant, I found an interesting utube video on cleaning up a Streptocarpus plant for a show.
https://m.youtube.com/watch?v=...
About 2/3rds through, they note that the plant in question has a spot of mildew on the leaf. They simply trim this area off and comment that in about a week to 10 days the leaf will grow back.
If you are getting mildew, you might be overwatering your plant. Again, avoid getting water on leaves. The following tube video might be of help.
https://m.youtube.com/watch?v=...
Streps are easily cared for, so long as you know the drill.
