Laying age depends on the breed, and often the season too.

Wether to clip wings or not depends on your set up & if you have enough land or just keeping them through winter inside? Our ducks stay home so we don't clip them. In my limited experience a clipped bird is a dead bird if any predators can get to them. Your set up sounds like that would be probable, though I can't say for sure having not seen it. We have weasels & they get through some pretty small cracks.

We've had pekins several years ago freerange through the winter no problem, never had a need to give them a heated environment. Your crested drake will probably need protection, if your winters get cold.

These are my first thoughts based on the info provided, a more detailed description of their living quarters might help. Hopefully others will give ideas for penned ducks.

Does this mean there's no longer a building above this basement? Is there good drainage in the floor so they won't flood -- ducks love water, but they need dry land, too!

I would be very cautious with the greenhouse idea. Keeping birds out of weather is much more important than heating them. Will they have the opportunity to go outside?

You should leave food available to them 24 hours a day, and they need drinking water they can dunk their heads in if they don't have a pool.

My Pekin hen started laying just before 6 months old.

Enjoy them!