All passive is the easier option if you want both stereo and mono jacks, and a switched stereo jack would probably be the simplest solution. When a stereo plug is in you'd get your separate channels, pull the stereo plug out and both passive circuits would go to the mono jack.

Your active plans are more problematic as you're essentially wanting to mix the output from two separate preamps into a single mono output when mono is selected. Power switching via the jack(s) for the preamps (as you normally do) plus having the stereo/mono switching and combining two preamp signals for the mono output makes things difficult.