HTML5 Boilerplate homepage | Documentation table of contents
<head>
.--
<head>
.The main advantage of placing it in the <head>
is that you will track the
user's pageview
even if they leave the page before it has been fully loaded.
However, having the code at the bottom of the page helps improve
performance.
One simple way is to use Initializr and create a custom build that includes both HTML5 Boilerplate and Bootstrap.
Read more about how HTML5 Boilerplate and Bootstrap complement each other.
No, same as you don't normally replace the foundation of a house once it was built. However, there is nothing stopping you from trying to work in the latest changes, but you'll have to assess the costs/benefits of doing so.
Please ask for help on StackOverflow.