Hi Mike,
First of all what’s this’ll about? You need to patch 3 core files but you don’t provide the numbers of the trac tickets? I’ve never seen that before. Can you provide the tickets numbers please?
You don’t need to patch the files yourself; s2Member handles that automatically (from the Multisite WordPress Patches page: “you will need to use the automatic patcher below. All you do is check the box & click Save.”).
When you check the box and hit Save, s2Member patches the files for you. You don’t need the WordPress Trac tickets to patch these files, but if you’d like to take a look at the tickets for your reference, I will contact Jason (the s2Member lead developer) and see if he can reference those here for us.
Can you answer me one question specifically? How does the plugin control the number of sites a member can create?
This is determined by the s2Member Level the user has been granted. You can define how many sites that user can create based upon the level. Please see screenshot below:

Regarding the Unlimited-Site License vs the Network Support License
In your original post, you said,
I want to offer the ability for the client to purchase the ability to add more sites under the same conditions.
That would imply that users are creating and administering additional sites in that multisite network. As per the “Definition Of A Multisite Blog Farm” at the bottom of the s2Member Pro Prices page:
In the case of a WordPress® Multisite Network operating as a Blog Farm, you WILL need our Network Support License […] If you operate a WordPress® Multisite Network, and your Network DOES make it possible for users/members to create child blogs or sub-sites (in any way), we consider your installation to be a Multisite Blog Farm
So if it is the case that you will be allowing users to “add more sites” and to “run”‘ those additional sites, you would need to the Network Support License. Even if the users are paying you to create additional sites for them, your site is still considered a “Multisite Blog Farm” and you would need the Network Support License.
A Multisite Blog Farm (in the eyes of s2Member®), is a Network that makes it possible for its users/members to create child blogs or sub-sites; where one or more of these child blogs or sub-sites is administered by a user/member (e.g. if you offer blogs or sub-sites to your customers, for free or otherwise).
Does that clarify things? If not, please let me know and I’ll see how else I can help. :)