3. I know I am slowish especially on hills (up surprisingly) my commute is usually averaged at 20 kph rolling average. Do I try to improve this? Is that even possible in 4 weeks? Or do I just trust to my ability to keep going through the night hours.
20km/hr is pretty slow but the traffic lights may be the cause, most Audaxes are on roads that allow you to make decent progress.
A cafe stop makes a serious dent in your moving average, in fact moving average is almost irrelevant in Audax, it's a game of overall average. Eg. 50km/hr moving average is useless if you stop for an hour every few miles.
I've never used moving speed as a guideline. Audax allows 6hrs 40 mins per 100km. I use 5hr 30mins per 100km as a goal, including all stops. Sometimes circumstances mean it slips nearer to 6 hours per 100km but that still puts time "in the bank".
That's fairly achievable and allows time for punctures.
If you don't "dick about" in cafes then 6hrs/100km is pretty easy, i.e. a 24hr/400km.
So just make a note of how many kms you should have covered at a certain time of day, maybe in 50km increments, just to keep yourself roughly on track.