re: Bundt cake questions - I can help somewhat.

SANDY:
Good Morning. YES, Sandy, if you open the oven sooner than 13/15 minutes you placed the baking pans into your oven you will interfere with the chemical leaveners. In other words they will fail.
Sandy, if you want a moist cake--that is easy enough to do. You can simply substitute up to 25% of the sugar amount in the recipe with honey, oz for oz. But in doing this you must reduce the oven temp. by 15/20 degrees due to the carmelization effect. With the excess batter, you can make muffins or smaller bundt cakes. You do not need to bake everything all at once. You can place the un-used amount in the fridge & then fill your secondary pans in after your 1st. load is baked. "CAUTION". If this recipe uses baking soda you then must consider adding a pinch or 2 of same into the batter before baking. Why, because once soda is mixed in with wet ingredients it begins to function if it's in the fridge it is disappearing into thin air, that's why. It must be replaced.
As far as why the cake collapses on you while cooling I do not know why but make certain you are mixing the ingredients long enough to beat air into it. It may be a mixing problem. Well I hope this helps. I woul like to encourge you to bake from scratch you will get a much better baked product & it is much more fun.
Good luck & have a nice day young lady.
~Chef Dunask.