I/m about to install my Chameleon emcomm II. Hight end will be around 18 ft.
1) how low from the ground need to be the match box
As high as possible will probably give best results.
Obviously, if it is more than 18 feet than it is no longer the low end
of the antenna.
Ground losses increase as you lower the feedpoint. You have to
weigh that against installation convenience. That's a personal
decision.
2) does is better to have multiples counterpoise and if yes how long or only one
The best counterpoise is probably a wire the same length as the
radiator running in the opposite direction and elevated at about
the same height. Basically a dipole.
Or one such counterpose / radial cut to 1/4 wavelength for each
band you want to use and elevated in the same manner. They
don't have to run straight off the back, but can angle as needed
to tie to available supports.
That assumes that a primary objective is radiated signal strength.
Other operators may have different priorities.
3) I will use 100 ft coaxial.
That should, at least, reduce the stress on your tuner by lowering
the SWR it sees (due to losses in the feedline).
A couple of the local ARES operators use that antenna, and they
seem to really like it. On one recent operation, however, they
complained that band conditions were so bad that the net control
couldn't hear them. Those using portable dipoles didn't have that
problem, however...
One of them did observe that, when he mounted the antenna as
a dipole at 20 feet, it worked a whole lot better!