How/should I modify my ethics review proposal when asking questions not stated in the original application?
The simple answer here is that if you go outside the scope of what was approved in your ethics clearance, then you are doing something for which you do not have ethics clearance.  Thus, the answer to your question depends on the specifics of what you have described in the application that was approved.  If your approved application represented that you would use the specific base questionnaire that you submitted, as written, then you will need to amend your application if you now wish to deviate from this.  If your approved application merely represented that you would ask questions within a certain field, with the questionnaire serving only as an example of the type of question, then you probably have scope to deviate a bit while remaining within the scope of what you described.

For this reason, it is a good idea when you draft an ethics application to anticipate possible adaptations you may wish to make as you progress your research, and describe your proposed activity in a way that is broad enough to allow you some "wiggle room" within the scope of the approved research application.  This is a balancing act --- if you give too little detail then it will be too vague to be approved, but if you are too specific then you are imposing constraints on your research that you may wish to change later.  In my own field, of statistics, it is common for an ethics application to give some information about the proposed model/method for analysis of data.  In this case you need to make sure you frame this broadly, to allow yourself the discretion to adapt your method of analysis to the features of the data when it comes in.