Good questions:

Standard issue is two tracks on either side, as you are aware. I replaced my original ones with Shaeffer tracks, but mounted largely where the old ones were. My only criticism of the original tracks is that they were too short. The forward track I use with the 100% and the storm jib and I would like it to probably be a foot longer in each direction. If you can't find the Shaeffer tracks that I mentioned above, let me know and I'll look them up for you.

The genoa track (and my genoa is a 170%), could be six inches longer in each direction.

I just find myself putting the blocks at the extreme ends too often, which infers that I need a bit longer slider.

I don't know about putting them outside the lifelines. The underside of the gunwale is a terror to work with, and I don't know how you would secure the tracks underneath without a lot of effort. Additionally, I would be concerned that you might pull the deck off the hull where the tracks are mounted, and take in water. If it only cracks that seal (and I have had this problem) the crack becomes a devil to find, is probably not visible, and is always leaking. Perhaps the one major defect in the Ariel that I am familiar with is the bond between the deck and the hull. I would not want to place stress on that bond.

The forces on the tracks can be immense. I have had a track pull through the deck (and now have substantial backing plates).

Hope this helps.