Use of an em dash.

Firstly, can a comma precede "and" when it is a parenthetical add-on, for example "Abby gave me a tip, and expected a tip"? Should there even be a comma here?

So, let's say you'd like to include an em dash for added emphasis here. If it is a parenthetical add-on, then we have:

"Abby gave me a tip, and expected a tip" -> "Abby gave me a tip – and expected a tip"

Is this correct? I understand we could just write "and she expected a tip" to create and independent clause and therefore avoid any problems.