For the first point, I guess the amount shown on the submission page is already adjusted based on the real completion time that similar studies like yours would take to be finished. For example, if you put 15 mins as an expected completion time but in reality, those 15-min surveys actually only take 10 mins to complete, then this would bring up the average amount you have to pay to the respondents. But rest assured that you only pay for what you have. So 7.5 is 7.5. You will not be “forced” to spend 9.60. This is just my guess though haha!
For the second point, the answer would be NO. The participants will be paid the exact amount you have set up in the system regardless of the time they take to complete your study.