See my answers inline.
Please ask the customer to post directly here any other questions/concerns and we'll do our best to answer them.
Also please remember him that there are video tutorials available for him to get more familiar with our product.
What they would like to do is have an easy way to pull and put data from the forms app into the backend database which is a very complex schema with many linked tables.
QA pull and push data from and to teh db using the SI suite web parts. As long as these operations are defined and built (stored procedures, web services)
Didn't really see anything in the documentation that shows how the data connection works.
You can connect to multiple systems in the same time and make them available. See the Configuration Editor User Guide section Connecting to External Systems.
They have many SP sites and have a need for each customer (user) to use custom forms to pull, put and update data into the backend SQL DB.
You can expose multiple systems to users and you only need to define it once per each web application. More details can be found in our docs (user guide).
At the same time they would like a solution that their staff could use to build forms for each customer without having to engage engineering for custom code or data connections.
There is no need for custom code in QA. As long as you define your operations (stored procedures) the users can build the forms based on that. Regarding the data connection see above, the data connection is defined once and exposed to all users/sites.