Fighting Question Slides bugs. Any tips?

That version has still more bugs than version 11.5.5.

Rod already gave you a lot of suggestions. Personally I also suspect you are missing some ways of customizing. You are creating a fluid boxes project, a responsive project. The master slides for that type of project are set up carefully, having a static fluid box for the feedback messages. Maybe this older blog post, where I explored those master slides could clarify

Quiz Slides with Fluid Boxes - Captivate blog (lilybiri.com)

As for the buttons: quiz and score slides only support the three older types of buttons: text (default), image buttons and transparent buttons. You get the best result to customize when using Transparent buttons, because they have almost all the functionalities of the Shape buttons, with the exception of not being able to time them for the rest of the project. A complete description of all the button types in your version is available in another blog post:

Overview 6 Button Types - Captivate blog (lilybiri.com)

All items used in Quizzes have object styles which you find in the Object Style Manager in a dedicated category. Please have a look. 

It is also possible that something is awry with editing the fluid boxes layout? Please, be not offended, but you tell that your previous version was 8 which didn't yet have the Fluid Boxes workflow. Did you edit any of them? The first blog post refers to those master slides, which are completely set up for the different types of question slides. This refers to your complaint about the progress indicator being wrapped. Did you edit its starting font size? You may prefer to decrease the minimum font size which is at a much too high 14pts (when expecting use of phones, I lower it to 8pts).

What you describe do not sound like bugs to me.  The issues sound more like your project Theme and Master Slides have not been set up properly.   

Button states can be very powerful, but if clicking on a button causes the text to be left justified, that would tend to indicate that the states have not been set up correctly. By default the Down state of any button is deliberately set to appear slightly smaller at runtime to give the effect of pressing the button.  That may be what you are referring to.   If you do not want that feedback change when clicked, you can remove the Down state.

Did you use the Master Slide layouts for quiz question slides to try and change the formatting of the buttons, or did you just do it on individual buttons on the resulting slides?

The Question X of Y caption that appears on quiz question templates can easily be made larger to avoid text wrapping.  Just select the placeholder for it in the Master Slide and change it's width in the Properties tab > Options area.

I would also recommend you set up a number of custom object text styles that you can then apply to all objects that should share certain formatting (e.g. quiz question buttons).  That will give you much better control over look-and-feel, plus it then allows you to change formatting for all objects attached to that style by just altering the settings in the object style.

If you haven't used Captivate since version 8, some of these features may be unknown to you.  One caveat however is that the project you are currently creating in Captivate 2019 will not be transportable or updateable to Captivate 13.  So, that may be something to consider.