Anyway, with multiple cars, there's no real extra work needed; only one car can really be in the Lead, but if two or more cars wanted to travel side by side in the Lead (say, to block the road or something), I'd allow it as a simple Stunt with their Driving skill. If both drivers make the roll, they can share the Lead, driving in tandem without accidentally smacking into their partner.

If you're not in the Lead, the rules don't really make a big deal out of where you are. I would just assume if they're not trying to take the Lead, your PCs can pretty much be wherever they want in the chase pack. Zipping around between mook vehicles like bumper cars can be fun; all you have to worry about is making an attack roll (using the Driving skill, or having passengers shoot), and run combat as normal.

When forcing another guy off the road, I'm assuming you're talking about mook cars. That's still just an attack; if your outcome is high enough, the mooks spin off the road in a glorious cloud of dust and squealing rubber. [If you're trying to force Named guys off the road, well, then you just need to follow the regular combat rules. You can't really automatically force a Named opponent off the road until you total their car's Wreck... unless the GM thinks it's cinematically cool.] However, remember that whoever's in the Lead can escape the chase altogether by making three 'Escape Rolls'. If you managed this, you could easily describe your escape maneuver as finally running your opponent(s) off the road.

Finally, no, I don't add Outcome to Wreck damage. The rules don't specifically say this, but it's implied, I think, by the fact that the rules say "guns do one point of Wreck" etc., with no mention of adding Outcome in any of the damage notes. [Besides, if you look at the Wreck points of the vehicles listed, adding Outcome would wipe out a car rather quickly, and I don't think that's the idea with Named characters' cars.]

Hope you enjoy your chases... I *love* chase rules!