Ikaru may not be an expert on the series, but I am.
Let me just point to a few things. About the "read the LN" ending of the second season, there is a reason why it was done that way. If you look at the PVs for season 2, you'll notice something interesting…action scenes for two characters who never had any actions. The PVs gave a quick preview of Terra using Execution of Light in the Palace of the Popes and Acqua swinging his weapon of choice in District 22, implying that the second season was going to originally adapt volumes 7-16. That's right. NINE Volumes plus 1 SS volume all in the span of 24 episodes. You can imagine how rushed the season would have been if the staff had gone through with that. At some point, the staff decided to cut down the volumes adapted to 7-13+SS1 and SS1 is pretty much the kickoff point for the GRS Arc of Index.
Let me also point out that when Index's first season started, there were already 13 volumes written and double that amount by the time Index II started. Let's just say that the author is really good at putting out great content at a nice speed.
Needless to say, the Index anime, I think, is great on its own. Plus, it did a great job of making me actually want to read the Light Novels. However, I will say, as someone who has read each volume of the novels 2-3 times at least, that the anime is sorely lacking in something the novels had. Character development for certain characters--Touma, Index, and Aureolus, for example, feel quite underdeveloped in the anime. However, the novels really take these characters and develop them well. The way Touma's character develops in both Volumes 1 and 2 for example are really powerful and are compounded by the later arcs. But I digress. I could probably write a full essay about this but that would be a long and daunting task and would probably be beside the point.
That being said, it is a wonderful series with a wide world to explore and speculate about. So, if you have any questions about it, I'm your guy~