Is reusing keys for CBC and CBC-MAC secure when using encrypt-then-MAC?
No, it is not necessarily secure. Here is a simplified example of why not.

Assume one block zero messages are encrypted without padding. The ciphertext is $I||E(I \oplus 0)$. The MAC value is thus $E(E(I) \oplus E(I)) = E(0)$. So regardless of the IV, the MAC is the same for all such messages. So if you encrypt several zero messages you can leak that fact through the MAC.

When you take padding into account this exact issue is no longer possible, but you should see how it is possible for the MAC and encryption functions to end up using the same values.