I run a self made OpenVPN Server on Linux (Fedora) with Windows and Linux clients for my home/office.
I see no issues in the OpenVPN or even a choice of Linux distribution with good GUI (which you require) like Fedora.
Being a Novell Certified professional, I clearly understand Novell Netware domains. In case you want to implement a Windows kind on a Domain services on a linux server, then also no problem.
My commitment here is to complete the work for sure, irrespective of whatever time it demands, or however more sessions you want for training after commencement but before the completion of work. My bid remains the same. The time frame of 20 days is on a higher and safer side. It may get completed sooner, much sooner.
To be reasonable, you can pay for any extra hours of hand holding which you may ask for later separately.
If you want, we can run a test / trial with my OpenVPN server here, as a confidence building measure, before you assign me the project, which I know I can do it for you.
I have noted the problems you have faced so far, I believe you will have a completely satisfied and different experience with me.