I have seen fish, brush, etc. in 2D and not in SI a lot of times.
1. If your sensitive is too low in SI it will be hard to see in the water column.
2. The best color for me if I want to see things in the water column is Amber or Amber II.
3. 2D as was stated only covers 1/3X water depth if set to 200kHz, and 1X water depth if set for 200/83kHz. The SI water column is 1X each side of the transducer, so that area was covered.
4. If I remember correct Chart Speed is global, so if you set it at 4 all sonar will be at the same chart speed.

Just my $0.02