In the timeline, right click on the audio track and select audio gain. This will pull up the audio gain window where you can just type in + or – X db. Or you can enter the exact db you want your audio set to.
Here’s my workflow for AVCHD. Copy the entire contents of the card onto your media drive. You can create folders for different cards but the entire card needs to be copied into the folder. Inside Premiere use the Media Browser. Navigate to the clips and the media browser will connect the spanned clips and will show you thumbnails of all your clips. Select the clips you want and import.
The audio mixer is for track based mixing/keyframing and not clip based. Adobe expects you to mix your audio with the mixer once your video is locked down. If you want to adjust audio per clip then you have to use the pentool and create clip based keyframes.
What I mean by “outside” is sending to someone else to work on it. Like color grading. But if you are going to complete the project on your machine using the Adobe Suit tools then I would stay with native files and not transcode. Just to be clear I am not an expert with Premiere. I only recently switched from FCP7. I have done some minor color correcting with Premiere’s 3 way color corrector effect with my native AVCHD files and it’s worked as expected. No issues using the native files. As a test why don’t you import a couple of native files into Premiere and start playing with the color correcting tools and see if you are happy with the results. Not having to transcode saves time and lots of drive space.