First off, make sure you have proper storage containers or sleeves for your comics. Keep them away from moisture and excessive heat. And be careful when flipping through the pages to prevent any tears or creases.
The number of stress marks for a near mint comic can vary. Different graders might have slightly different criteria and might count the marks differently based on their severity and location on the comic.
Hard to say. You could ask around in your neighborhood or search on popular mapping apps. Sometimes, local shopping malls or comic conventions might have manga stores too.