Personally I think that it added to the show. I felt more sympathy for the characters and it gave a reason for their actions. The only part that I got distracted on was when Kaede, not sure on the spelling, snapped becasue I couldnt take the scene seriously. I kept picturing her as Rena form Higurashi and all of Renas lines kept popping into my head while I was watching that scene.

Made the show all the better!

^ I completely agree with that, It shows their character in different ways, Making everyone of them unique! It was a very clever touch to the story!

Coming from the VN background I have to point out that the concept of mental illness was actually played more on an interesting note than it was in the VN.
Kaede was played no where near as crazy in the VN style as she was here and I think it is because of the production teams choice to go with the Asa route since it was in fact Kaede who introduced Rin to Asa in the first place so they were playing off of that fact where Kaede was also pissed at herself for having introduced them in the first place.
Primula wasn't mentally ill at all, she was after all a miracle formation of combining God and Demon efforts to basically if you want to call it make life. This actually is to play with Rin being the cause of emotion which is the one thing that was missing from Primula all the years she existed. I prefered the VN route of Primula over the way they chose to present it in the anime.
Asa has the same problems just couping with who she is, blaiming her mother for her physical health and her desire to be just human. (as her body is human and cannot contain just magic levels well, where Primula's body is designed for that yet the emotions she feels lets that leak out of control for a bit)
Sia, not a mental illness, as she is after all not human and is holding the soul of her sister in the same body. This is not multipersonality Syndrome but rather more along the lines of conjoined twins that share a single body.
Same concept with Nerine, but in this case she lets her other half go in her memories.

In truth there really isn't a mental illness expansion from the VN but more for entertainment purposes was exploited a bit to make for more interesting television.

As for your wish #3 Saplock, they wouldn;t have had the money to pursue such an investment. Also, that is what the VN is for. If you want the other paths purchase the VN and go that way. Otherwise for anime sense it is impractical to make seasons for each girl as it also causes you to either repeat story points or neglect a character for a long period of time.