Hello,
My name is Janos and I'd like to help you on this. Thanks for the drawing and explanation - I understand data access and security is the question here. I have a vast experience and can give you an API that hides this and decouples it from the application.
Please can you contact me via the freelancer chat to discuss? I need to better understand your exact needs and can then quote you a time and cost for help (consultancy) or actual development as you may require.
About: I have over 12 years of experience with .net & SQL development from both a corporate life (in the last 10 years I was leading development for SEWS-CE a major automotive company's bespoke ERP, MRP & EDI systems) as well as freelancing - please see my previous works here on freelancer for references. As for development: I have 2 other developers I work with, so a team of 3 and we can tackle any .net development including projects of this complexity, taking you through the entire process from requirements analysis, scoping, carrying out development, going live and providing support and continued development at any time.
I look forward to working with you on this interesting project!
Kind regards,
Janos