1) No, you can still edit the user Registration solution pack using SA2
2) The advantage to using SA to create your registration system is that you get to create the registration you want. read my response to #3. Thinking in terms of the bird house example, user registration solution pack is similar to buying the bird house from the garden store, you have to choose from what they have available, it may not be exactly what you want. Using Security Assist, Universal Email and DataAssist, you can create the registration system exactly as you want it.
The solution pack licensing is also for a single domain, whereas you can use the tools to create your own registration system from as many sites as you like.
3) Well, User registration Solution Pack is not a tool, it is a pre built application. It is the result of using the tools to create an application. Think in terms of the bird house that is created using the saw, hammer and nails. The saw hammer and nails are the tools, the bird house is the result.
you can use the tools such as security assist and universal email to modify the user registration solution pack by editing the server behaviors.
4) WE do not currently have plans for a User Registration Solution Pack V2, but it is certainly a possibility, you can post to the wish list to have the idea considered by the engineering team.