replied on January 19, 2017
From a cursory attempt at reproduction, I cannot. I set up a user task with options "Approve" and "Reject", and specified that the form should be read-only to the assigned user. The form itself had a single single-line field and no theme, rules, or custom code of any sort. After typing into the field and submitting, for the user task the form was read-only but I could still add comments and choose either Approve or Reject.
The form and process were originally created in Forms 10.1 (can't remember which update) though I did only add the read-only setting after updating to Forms 10.2.
In your form, do the approve buttons seem disabled, or does it just seem that nothing happens when clicking them? If they seem disabled, are you able to enter text into the Comments field? If they do not seem disabled but nothing happens when you click them, check the browser console (F12 > Console tab) for errors when clicking the buttons.