Both of the modifier keys (Snap and Pop) work just like Shift and Control. You hold them down to get the other layer. On mac keyboards you can't count on more than 2 keys pressed at the same time so a Shift + Snap + A might (or might not work). I had a few failed attempts at combo modifiers before I decided they wouldn't be reliable enough (at least not on my mac pro).
Yes, if you hold down J and Semicolon and then hit the a button on a macbook pro you will notice it isn't picked up. It is a hardware limitation on most keyboards. See "n key rollover" in google for more details.
On the pop layout I have a bunch of keys that are used for shortucts. You could remap any of those to CTRL + ALT + Shift + Insert or whatever other combos you hit often. Also, tying numbers on crackle is as easy as typing capital letters so I don't really miss the dedicated numpad (but no 10 key typing of course).
I just can't stand the idea of walking around with an extra keyboard with my laptop. And the few small keyboards I've tried still don't place all of the special characters in easy to remember places.
I'm not sure if Dvorak is really faster, but I do think it keeps your fingers on the home row more. At this point the main reason I prefer Dvorak is that it lets me have more modifier keys on the left side of the keyboard (without moving to the number row).