Sounds like you need a Shimano M324.Its a normal bearcage pedal with a cleat on the one side.You can use normal shoes on one side (with a toeclip if you desire) and then flip the pedal over for use with cleated shoes.Seem ideal.We sell a lot of them.
Thanx Max
You answered your own question a while ago:
Maybe I'm just overthinking the whole thing.
Go Buy some M424's (cheapest platform SPD Shimano do) try them for your "Normal riding" and give them a go for DH, you'll probably get along with them fine for "Normal" use (I assume by normal you mean some distance work and XC bimbling/Trail riding) and probably ditch your toe clips...
For DH you'll find them marmite like, either you like the security of keeping your foot planted or when in "panicky mary mode" you'll find your terror exacerbated by not being able to dab a toe, the platform can help a bit as you don't have to clip in when you put your foot on the pedal but the grip isn't wonderful, if you really can't ride DH with SPDs then go and get some Flats and a good set of skate/BMX shoes and change pedals according to the days planned use...
Your 3 pedals in one idea is frankly toss...
Good luck...