First question. I would like to use the lines as my ground return and
remove the 2 AWG wire I have now. Good idea?

Second question. Which would be better, or possibly which would be worse
and why? Copper or Aluminum.. Aluminum would be lighter but copper offers
a better ground return(less resistance). I have a big engine and the
starter needs all the current I can muster.

I would not substitute the brake line for the #2 ground return. Beware of
corrosion that may occur in any connections that join copper and aluminum.
There is a potential difference, and the voltage created causes corrosion.
In some cases, the corrosion could continue until something bad happens -
including loss of electrical system. Any weight saving would be false
economy.

I would use copper rather than aluminum. Aluminum is less weight, but it
requires more preparation to connect. Keep everything in copper. If copper
touches aluminum, you will get electrolysis unless a special goop is used
with an approved connector.

Colin