I understand your concern, but I don't think that a bottom drain is designed to deal with leaves and debris that sink to the bottom. First of all, you want to avoid having all that debris sink AT ALL as it adds nutrient load to the pond that you don't want to have to deal with. It will also stain your water with tannins, which isn't harmful, but many people find the discoloration undesirable. But most importantly, the leaves that sink aren't going to move easily to a bottom drain - they get wet, they get sticky, they get heavy. Basically, they sink and they just lay there until you stir them up. I don't see a bottom drain being of any help for that kind of organic debris once it gets to the bottom. You'll still be in there with a net scooping and trying to get things moving. A skimmer is a better bet as it keeps the leaves from ever sinking in the first place.
A bottom drain is often called a "koi toilet" for the very reason that it's meant to carry fish waste out of the pond. People often use them when they have a heavily stocked dedicated koi pond so they can move the waste filled water to a settlement chamber before it goes to the filter. They let the solid waste settle out of the water column and then filter it. You definitely don't want to pump debris filled water from the bottom of your pond directly into your bog. You will clog up your bog in no time.
Just some thoughts while you are still in the construction stage.
