Fixable?  

Nope
You would need another donor chair and some magic. It's not worth the time or money to try and mess with this. If I saw this at a yard sale, I'd give them $5 just to see if I could get the mounts off.

Why would you even bother?
At any given moment, many hundreds of intact examples of those chairs are for sale. You should have no trouble finding one that isn't already broken and worthless.

McGruff says:
Take a bite out of crime.

Only because I
1) am cheap, and
2) enjoy a challenge.
I also like bringing back an object that's been left for dead. But, as you say, it's probably a dumb idea to attempt this one.

I found
a Herman Miller shell chair on the side of the road. The back was broken off and it had missing feet. I took a saw to the broken edge, repainted the legs, and got rubber feet for it. Now I used it every day when I put my shoes on.

I know you all like Haiku's but...
I like limericks better.
There once was a broken chair
It suffered a crack and a tear
Push came to shove
It was given some love
And now is given much care

how about bite marks?
I already tried to reconstruct broken off pieces like that, but you ll never get the strength within the material so that it will be as durable as before.
I once thought about just taking a saw or a rasp and just making the form of bite marks out of it. Delicious Design it is, isn t it?!?
Got the idea from the flatware my sister gave to me as a birthday gift.
http://www.whokilledbambi.co.uk/2010/12/mark-a-reigelman-ii/