Is trivial to mild mitral regurgitation usually audible to a cardio with a stethoscope? Is this something that normally progresses?

Mitral regurgitation. Audible sound depends on amount of turbulence caused by a lesion. Generally trivial MR causes little or no murmur, mild MR might cause some. The minimal MR lesions usually don't progress unless endocarditis occurs but progression is not impossible.
Depends. Depends on the severity and how well trained the air is. Progression depends on the etiology. A lot of people have mild regurigitation that's physiologic. However if it is due to mitral valve prolapse that could progress. .