A quick answer to your questions:
Using the buttons, you can create a nice and quick navigation. You can also combine this with a rectangle element behind to make it look like a bar. Also, every element is effectively a button. You can switch on the button controls by going to Edit->Show Button Controls. That way you don't need to use the default buttons. You could use text, or a rectangle, or ellipse.
You can click the scale settings to make the document flexible (width only)
that way you can choose the following pattern in the Metrics inspector to make your element expand the width of your document and also centre it.
Your scene dimensions are editable to whatever size you want. Just type in the dimensions and you're good to go. The presets are just that, "presets" as in they are templates that you can use.
If it is checked (as in the above picture) then the dimensions will assume their values throughout all your scenes. If you need a specific size for mobiles as well then you can create a new layout and change the dimensions for that specific layout.
There are many posts here that have many examples and templates for you to look at and pick up some techniques as well as what has been suggested. The documentation is a great resource (not complete) but great for the basics and a few advanced things too.
Here is a simple template to get you going
basic-with-navbar.zip (51.9 KB)