How can I route a toilet drain around copper supply lines right below the slab?
I hate to say it, but there's really no alternative to excavating this and rerouting the drain line. The reason offset flanges are so limited is that they are MUCH more prone to clogging. I've heard they're not even allowed by code in some areas for that reason. 


  I don't trust Shark-bite below ground or on old possibly out of round pipes


Shark-bites are fine below grade. If your copper pipes are really so bad off that you can't shark-bite them, you've got larger issues.