ISO: Bundt Cake Questions

I like to make bundt cakes, often the type starting with cake mix and a box of pudding as a base, and then experiment with additions and/or alterations.

1) Occasionally, the sides of the cake seem to collapse onto themselves as the cake cools, making a mushroom-like effect. What causes this and how is it avoided?

2) Mix cakes make enough batter to fill a 12-cup pan, but many of my newer designs are only 9 or 10 cup capacity. Any suggestions for the leftover batter? Would opening the oven to remove cupcakes, for example, harm the main cake?

3) No matter what I've tried, additions such as blueberries, raspberries, raisins etc settle to the bottom as the cake bakes. Tastes good, but...

Any other tips would be greatly appreciated --- butter vs. oil, number of eggs (I've seen recipes using 2,3 or 4), benefit of pudding (of lack of benefit). I prefer moist cakes with a nice body (not too light).

Thanks!