10-15mm is larger than I would expect to see for a drain hole. Typically that single hole is where you would screw in the cable guide and it is much smaller. You can often get away with using an adhesive for the cable guide and the tension from the cables themselves will help to hold it in place as well.

You don't usually neeeed to chase and face the bottom bracket shell. If the bottom bracket can thread in without much force from the tool then it is surely fine and chasing shouldn't be needed. Facing often isn't needed either. If you want to be very precise then go for it but otherwise it isn't usually necessary.