I know how frustrating that can be! It shouldn’t be a problem. You’re interfaced collar will be the top collar, but as long as the interfacing fused to the fabric smoothly it really won’t be an issue.
And lots of people machine-stitch the collar edge. I like to hand stitch because I can control the edge a little better, but I sometimes go back and machine stitch afterwards, too. It’s really a matter of preference. I often tell students that there are many, many ways to sew. Find the method that works best for you and don’t be afraid to use it. It’s not cheating; we all have preferences, and there is no correct way. I try to give as many hints as possible in the instructions so that someone who is new to sewing will get good results, but that doesn’t mean you can only do it my way!
I hope the collar turns out well, Kelly. You can also use the steam of your iron to help shape it.