Long time lurker here...
I just wanted to say that I think the changes look good. Shared mana pool might work out pretty good. But moving the elemental books to the research tree bothers me and could be good or bad depending on how it's done.
I like diversity in a game. The more the better. Makes for better replayability.
It may work out pretty good if, for example, you made each consecutive elemental book cost substantially more to research.
That way, you can choose which patch you want to take after you begin.
For example:
Have a fire shard nearby after your start? Great, I can start research into fire magic. Or maybe you had your heart set on earth magic and made the tuff decision to just use the fire shard as an essence boost and hope you find an earth shard later.
Now, after I research earth magic I can make another tuff decision.. do I spend my precious research on getting fire magic to complement my earth magic? I already have a fire shard..
Well, (and this is nowhere near balanced, just numbers thrown in to get my point across so as to understand) I spent 25 turns researching earth magic, so now it might take 75 turns researching fire magic using my 1 point of research that I have if I wanted to also add that type.
Or I could wait until later to research it.
Now turn 200 I find an air shard.. do I want air magic? Well it takes even looonger to research. 150 more turns... and so on.
At this point it is not really worth researching it because I am using my time on that instead of civics or warfare etc.
BUT, I have the option if I wanted it.
As I said the numbers could be adjusted to better balance. I was just brainstorming an idea.
The different civs would be, well, different in magic. If the AI can be taught not to spend 500 turns of research on the 3rd or 4th book of magic, instead of just specializing in 1 or 2 that is.
If I wanted to roll-play an archmage of incredible power with weak army units I have that option. Or perhaps I want to play a civ that is high in tech and military and complement it with only earth magic to make even tuffer troops.
Just want to have to make tuff choices with different play styles . 
Is that understandable? Sometimes I have a problem getting my point across. I'm pushing 40 and old age is catching up to me lol.