Assuming that you have recorded in the default “32 bit float” format, the audio may be amplified up and down as often as you like with no loss of sound quality.
If you apply the Amplify effect with the default settings, it will amplify to a peak level of 0 dB. I expect it will then be easier to see the high peaks. Before exporting the finished track you may amplify (using either the Amplify or Normalize effects http://manual.audacityteam.org/o/man/amplify_and_normalize.html) to whatever peak level you want.
We recommend the -6 value so you can go over by accident in the course of live recording without causing sound damage. Sound damage happens if a sound hits zero — goes all the way up. That can cause permanent damage and there is no recovery in post production.
What you have is a perfectly healthy recording. -6 is a target or goal, not a commandment or indictment of bad recording practices. I’m betting the passages sound fine.
The Wiki Feature Requests page shows 18 votes for horizontal gridlines. There may be many use cases for them. I favour an option to turn them on so that you get as many grid lines as there are ticks on the vertical scale. I also think like James that there should be ability to add custom, movable horizontal grid lines. I would envisage this as a permanent thin horizontal bar above each track when the grid lines option was enabled.
You could drag this bar down into any track and drop it to add a custom grid line to all tracks. Drag from the permanent bar to add another custom grid line. Drag the custom line up or down to move the custom line. Drag it off the track to remove it. James sees doing this with sliders instead (presumably at the end of the track, I don’t know).