...the situation is here. I don't feel that Tilde is being challenged enough.

It sounds like Arwen is a really smart cookie. To say that about a sheltie, an already smart breed, means that she's got to be interesting to try to keep up with, since the intelligence in a dog can be used for good (making mom happy with perfect heels, bringing things, etc.) or evil. (thinking of ways to get what they shouldn't have, for example!)

I agree that potty training was being hindered by a dog showing a bad example to an impressionable younger dog.

I'm glad that she's made progress.

Would it help to have the fur between her toes trimmed, do you think?