I didn't have Luf's kit, but vinyl corner bead for arches works well:
I just screwed it down so it "looked good" and went to work. I routed the middle lane and then went on the inside and outside to get my three lanes (I only wanted three).
Clamp your pieces together and route across the joints. I did this and the slots are perfectly aligned. Both bondo and Durham's Rock Hard work great.....I used bondo because it sets faster (just my preference).
Yes, paint both sides of the MDF just to protect it.
Use the brass screws/bolts through the track to give you the power taps and wire across the joints for certain power. Since my track gets disassembled and stored in the winter, I used the multi-pin connector method to join the sections and I have no problems.
Don't sweat it too much....even routed tracks can change pretty easily with bondo and a couple rolls of copper tape.