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I would have the whole head checked and all the valves/rockers checked on it. There is a good possibility that if one was adjusted wrong, they all were.
On the damage, you could have every valve bad. I would also check the cam for any gouging on the lobe surfaces to make sure they didn't dig into the cam from the misadjustment. Most likely the whole adjustment is bad and that is why you have the poor performance that you have.
I doubt there is anything wrong with the engine brake. They tend to have issues when items are too loose for it will knock the solenoids out on the engine from the rattling.
Make sure the new shop documents everything and does so precisely, then seek for the other shop to pay for those damages. Maybe even seek the advice of an attorney on the site here on what can be done. There is a good chance that on something like that, you can go after the previous shop for it. The current shop just needs to document specifically that the reason for the damage is due to misadjustment of the overhead. That is why shops carry workmanship insurance for issues like this. You lucky you didn't have a valve break off and drop into the engine wiping the whole engine out...
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