thepilotadviser,
Although I fully agree with the above posters re the usefullness of having a multi-cylinder engine monitor, I gather that you are mainly flying rented club or school aircraft that don't always have an egt gauge fitted? Assuming you don't have a turbocharger, the short answer to your questions 1 and 2 are "yes", to both. On a normally aspirated, fixed pitch aircraft at a high elevation field on a hot day, you won't hurt a thing by leaning until you see the RPM peak. Don't do that down low or with a turbocharged engine though.