[woah]

questions 1-4: "Semantic" css is a waste of time. Either you're putting information into the html or you're putting it in the css. Use what works well.

question 5: I'll usually put bootstrap into its own folder, have a style.less outside of the folder that imports bootstrap.less, import stylesheets with custom mixins after bootstrap.less, and import straight up custom styles after that.

question 6: go into bootstrap.less and comment out the stuff you're not using. For the js, use something like gulp-include to concatenate.'

question 7: Load my-mixins.less after bootstrap.less, copy and paste mixins from bootstrap into there, and modify them.