Questions abotu cancelling PIP

I’d like to cancel my PIP due to change in circumstances. I’d like to be as prepared as possible before calling the helpline and would really appreciate it if anyone had answers to the following questions:

1) I am doing much better and am top of my mental health. When the agent asks me when this change occurred, how do I go about answering this? With mental health, things aren’t as sudden as moving to another country for example, where it happens on a specific date. How do I avoid them scrutinising when I got better and potentially even trying to make a case for me paying them back for previous months?

2) What questions am I likely to be asked over the phone?

3) Can I ask for the assessment form to be sent to a temporary address?

4) How long does it usually take for an assessment form to be sent?

5) Will I need to send any supporting documents along with my assessment form from a third person or is my testimonial enough?

Thank you so much!

Note: I know some people might question my decision to cancel PIP and urge me to keep it, but I have made my decision and am happy with it

Thank you for your response!

If you look up 'Tried to cancel PIP' on Reddit, you'll see one of the respondents refers to an assessment form that is sent after the request to cancel is made. I've read the same thing from other people as well.