It's not clear from your post if you upgraded from an earlier version of Fusion (e.g., Fusion 6 to Fusion 8) or if you updated from an earlier version of Fusion 8 to the current version (8.5.6). It's also not clear what the state of your VM was during the upgrade or update (running, suspended, paused or shut down).
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I can say that Fusion 8 through 8.5.6 works properly on Yosemite as I'm using that combination now. Have you tried uninstalling before reinstalling Tools?
Things like the Task Bar should work even without Tools installed which suggests something else is wrong with the VM. Do you have a backup of the VM which you can test to see if it behaves the same problematic way?