To the first question I would use Cyano. The main issue with cracks is whether any glue gets into them. Sometimes you can work them or use a vac to suck glue in, but Cyano will just pour into the crack. It also doesn't seem to back at blocking itself, so it doesn't build a dam against further penetration. Hot stuff from Lee Valley (lots of other place also), the watery thin stuff is the ticket. just keep pouring it in until it backs up. You can often make a crack invizible with the stuff by polishing off with some fine sandpaper, but that is a slightly more advanced process. There are lots of other Cyanos that would work but hot stuff is the right cositancy and flashes the right way.
For regluing a clean sliver even white glue is just fine. Try fititng it in there a few times before you do it for real. Cyano is good if the stuff you use won't flash too fast. Good epoxy is also just fine, but most of the metal tube stuff is too thick to guarantee a perfect glue line. Cyano is nice because sometime you can insert the piece and wick the glue in there which pretty much gurantees you have the right fit.