customization – Linking a custom application form to database to manage a large user site?

Here’s a bitter scenario. Some experts could advice on this for a practical solution.

We are in situation that we are building a site with for a growing user base, transactions (user activities) and payments. The plugin and theme provides much of the features we need.

  1. There are 4 types of users. One type requires to go through a job application after registering. The plug-in doesn’t provide such functionality. So I am attempting to create a page with a job application form which the user can login, save/update and fetch application data multiple times until they submit it.

  2. I have created a custom page with job application form. However I have no access to the C-panel of the database and I am new WordPress. Since the plug-in provides the management of the user types, how can link this form to the current database and a specific table?

  3. How can I redirect the users after registration into this job application page? (the solution to this also serves to redirect an exiting user into the job application form)

  4. Do you think as the user base grows (so is the data), it’s recommended to have a separate database for users?

FYI: The webmaster (manages the server, hosting, and database) is from the last century … no change can be done on that, firmly objects to every attempt for even the slightest modification to the plug-ins. But I want to take a shot and see if we can achieve as it’s surely possible.

A workable direction to achieve this is appreciated.