Well the first issue can be solved by modifying your body css to:
As for the scrollspy problems, you’re not alone. A lot of people are having troubles with bootstrap 4’s implementation of it.
What I gathered is that you need to set an attribute called “data-offset” to the body.
I suggest you start looking into bootstrap 4’s scrollspy data-offset. Just google it, and maybe you’ll find a working example. I couldn’t.