You should be able to highlight the form in design view and re-edit throught the properties panel when the glowing area around the form is highlighted. This is a lot like the way spry validations work.
Some of the other things you are trying sound like they could be a bug. One issue is that this version only supports up to 2 columns of data, so maybe trying to add three fields to one line the way you want may not be supported without putting first/last name into a single column... I'd want to look at your result and better understand what you are doing to make sure, but that could be a limitation you are running into depending on how you are trying to squeeze three fields into that one line.
Adding fieldsets in itself should not cause problems, but not being able to add groups sounds like a bug we should look into. I'm going to open up a support incident to explore your situation further. You can access this incident at: