I believe it's only the area of overlap, yeah. Don't think you could exit a speed bubble, though, since the issue is (in my view) crossing over the interface between them - and you're sitll going to end up going through the edge.
Anyway
Electrum: it's based on what you are going to do. Kindasorta. Using it as a method of detecting traps, trying fifty hypothetical combinations for a safe at once... I could see that working.
Aluminum: Brandon said it purges other influences, and can eventaully be used to purify the body of other things as well
Gold: same as electrum, there's a lot of interesting things you can do in terms of information gathering if you make the potential past self less irritatingly random. Also, maybe if you can overcome the disorientation you could do something sorta like soulstamps.
Atium: Nobody's had the ability to practice with it. I assume that being able to regulate just how much future sight you actually needed at that exact moment, pulsing it on and off, you could probably dramatically lengthen the time you can spend burning it.
Nicrosil: If alumnium can let you purge internal impurities, you might be able to flare up the toxins in other peoples' bodies, instead of their metals.