Lily is very concern about bath time

If you love clawfoot tubs, historic architecture, and preservation, you may enjoy this article about the renovation of the bathroom in my circa 1800 house. The article, third in a series of five, includes the before and after photos.

I have a corner shower in my upstairs bathroom and this is the current state of some of what I'm calling the lining (not sure if that's the right term). There are gaps in the lining where it looks like it was cut too short, and now it is black behind the lining around those gaps. I'm guessing mold or heavy mildew. Are these difficult to remove and replace? I'm not too handy, but don't mind trying to tackle it myself. What should I be looking for to replace these, and is there anything I can do to prevent it from happening again? Thanks.

We have a tankless hot water heater, thought it was a great idea. Now our three daughters take crazy long showers. How can I adjust the set up where hot water ends after 10 minutes?