As you've probably already realized by now, everyone has their own methods for hatching. I've been hatching for years and I'll tell you what methods I use but ultimately you should go with your gut on what you think is best in your situation.
1) Most people will tell you to stop turning on day 18 but I have found that stopping a little sooner helps to prevent backward or upside-down chicks so I stop turning on day 16. The extra two days gives the chick a little more time to get into the correct position for hatching.
2) I move my chicks (all species) to the hatcher after external pipping has occured. I run a dry incubator throughout the entire incubation period until I see the external pip. I then move the eggs to the hatcher and get the humidity up to around 70%. This is how I've always hatched chicks and it works great for me but like I said, everyone's different.
3) I personally wouldn't keep them in with the goose eggs as the higher humidity could be too much for them. Goose eggs need to lose quite a bit of weight before hatching so I'd be careful with that. If your goose eggs look like they're losing sufficient moisture already, than a couple days with higher humidity may not hurt them. My geese are very valuable and extremely difficult to hatch so I wouldn't risk it myself. As for keeping goose eggs at higher humidity levels than chickens, I don't. Like I said, I run a dry incubator throughout the entire incubation until external pipping with all species. The only time I would advise against doing this is if your humidity is extremely low (under 25%). Mine stays around 26% - 28%.
4) This one is difficult to answer. I leave temps around 99-100 for hatching. If your still air varies that much...7 degrees up or down....I don't know if I'd risk it. If the eggs are just as valuable to you as your geese than I might suggest leaving them to hatch in the same incubator as the goose eggs, however, if your goose eggs are most important to you, I might go ahead and move them to the still air and try REALLY hard to keep the temp stable. That one is going to have to be your call.
Don't get nervous. I'm sure it'll all work out okay. If not, you're not alone. We all have had unsuccessful hatches and know how it feels. We're with you!