I too am exploring the opportunities promoted through the Police Now Graduate scheme and am in the process of applying so while I can't answer all of your key questions, hopefully I can help address some.
The application process for the current programme and 2017 tranche of graduates is open now (November) and closes in Feb 2017. If successful you will attend a Spring Induction in April and begin the Summer Academy in July 2017 - and as far as I'm aware by which point you will need to have achieved your undergraduate qualification so if you don't graduate until the August, next year's programme (2017) may be more appropriate.
The Q&A on the Police Now website is actually quite helpful in addressing some of the questions you've asked, for example where you might be posted (you can identify up to three areas registered with the scheme, again details of which force areas support the scheme is on their website). I imagine the final call from your three choices will be dependent on placement availability within each force area, but that's just a guess. Here is what it says
'How do you choose which force and station I will be deployed to? Do I have a say in where I go?
On the application form you can chose up to three equal force preferences, all of which you have to be prepared to go to. You can also choose to select ‘no preference’. The decision regarding where you are posted will be made in accordance with the operational requirements of the forces we are partnered with.'
Here is the link:
Good luck with your studies and if you choose to pursue the programme and hopefully this post will have been of some help.