In answer to your questions:

>re 1)
Generally, coupon codes – assuming we are talking about the coupons/discount plugin – are per site and are not “synchronized” – so to speak – across subsites
So a coupon code of some sort on site A will not be valid on site B (and vice versa). Nor will a code used on site A be marked as already used on site B

> re 2)
Wordpress multisite setups can share all users across all sites (somewhat anyway – i.e there’s only one users table , but there can be restrictions between sites for users. please see the wordpress codex for more info )

> re 3)
by default each (sub) site has its own order history / reports etc (although the master/parent site of a multisite setup can optionally – see wppizza->settings:multisite – display orders and generate reports etc from/including ALL sub-sites if so desired)

> re 4)
wppizza->gateways (for each site) allows you to enable/disable gateways (checkbox wppizza->gateways) as required for each site if enable sitewide.
Alternatively simply dont/do enable the gateway plugin for each site assuming you did not enable the/a gateway plugin sitewide to start off with

generally I would suggest you initially familiarize yourself with a multisite setup though – perhaps installing this sort of thing on a test server somewhere – as I am sure many things will become clearer when you do