1) The CD backing block is made of High Density Polyethylene. Most epoxies don't really adhere to HDPE (and for those that will bond, flame-treatment is sometimes necessary). So if I was really worried about it, I think I'd skip trying epoxy and instead buy another $10 block and drill again. That said...
2) The block is there to stabilize the bow eye ad distribute the load over a few square inches of fiberglass. If it were my boat, I wouldn't be concerned that the overdrilled hole might cause the block to pull through the bow.
I tend to be overly paranoid about all my repairs; and in this case, even I am definitely in the camp of, "Bolt it back together and go sailing."