1 - Yes, you can link to each other via My Disney Experience and the photos can all be gathered together there.
2 - It all depends on the time of day, the flow of foot traffic, etc. We've waited for a group or two to have their photos taken before us, and we've also just strolled up to the photographers and found them twiddling their thumbs.
3 - Yes, the photographers will take photos with your camera. But they won't do any setup, so if you have a nicer (DSLR) camera, you'll have to do the settings yourself or set it to auto... They also can't add "magic" to photos taken with your camera. But there are Magic Shots that you can request from most Photopass photographers. If they don't have a HUGE line, they'll happily pose you for a magic shot taken with their cameras. You never know who'll pop into your magic shot photos.
How to make it worth it? Make stopping at the photographers a part of your day at the park. You don't have to stop at every. single. one. But the more you stop, the more photos you'll have taken, and the more likely you are to get a great one. Ask for those Magic Shots. Are you going to do any meet & greets with characters? Assuming things are still the same as on my last trip (about 2 years ago), many of the character meeting spots will have a Photopass photographer hanging around. They are good at catching those fun moments of interaction between kids and the characters. (Plus they'll have everyone gather in the frame and smile for a quick posed photo before you all move on.) Also, when are you going? If it's during the holiday season, there should be a Photopass photographer hanging around the lobby at the Wilderness Lodge at least part of the time. They'll take posed shots of your family by the beautiful (and enormous!) tree. Lovely.
As for the rides + character lunch - if you order individual photos, I believe they're $15 each. So if all you'll want is the two ride photos + the character meal photo, you'll save by just buying the individual images.
I won't do a Disney vacation without Memory Maker. But I can't tell you whether it would be worth it to YOU.
