Open hammer pluck is really just using the left hand more. You need to be able to fit those extra hammer ons in between the slap and pop(s), keeping the right hand motion constant, as if the hammered note wasn't there.
I'm not very good at double thumb, but as I understand it: If you want to double thumb on the E string, your thumb should come to rest on the A string, having caught the E on the way down. It then goes back up through the E, using your nail. This is almost moving across the strings, rather than down into the body of the bass. Alot of books tell you that for "normal" slap, your thumb should be bouncing on the string you're slapping. Most of the really well known slap players don't do this, and go through instead (larry graham, victor wooten, marcus miller, to name a few). I find that it gives a fuller sound, and even sounds good on the D and G string.
Speed doesn't require any change in technique, other than making the movements smaller and faster. The more you practice it, the more natural the motion will feel and the less effort you'll need. Once basic patterns become easy, you'll be able to speed them up, and over the course of years you'll find it no trouble to play much faster, and wonder why you couldn't before
Emlyn
ps. IMO, because things are fast does not mean they are lacking in musicality or feel or anything else. If you can't play music with OHP or double thumb, then thats not a problem with the technique, its what you're playing with it. As for slap as a whole not being musical... well thats like saying drums and percussion aren't musical.
Free video instructions on bass guitar popping and slapping technique here! 
