Hi, A newbie here.Done exactly the same to mine about a month ago.Hand tightened bolts,waited for rtv to set and then gave the bolts a final tweak and SNAP!!.
Thought about drilling a hole through the bolt with a dremel and then trying to unscrew with a watchmakers screwdriver ,but have left it as it is with no problems.I will worry about it the next time the covers need sealing again.