Well, such an interesting topic, and also quite wide.

Ok, TWAIN, is an API for image sources, it can be scanner or photo camera. You just asking about working with TWAIN, without any explanation of how you are going to use it and what sort of devices you going to utilize.

So, I'll just share some of my experience. How I've used only scanners, different types and with different workflows.

• Server way. One or more stream document scanner (sorry don't know right term in English). Just any scanner which can work in network and configured to place all scans to some network folder or send my emails. Before sending some documents to this scanner, the operator should stick some barcode, on the title page, or on added empty first page. On the server side, we have used ABBYY Recognition Server, which just watches some folder, and can decode barcode and recognize text in the document, pack it in XML and place it in another folder. This folder was watched by Ensemble, where we searched for barcode in our system and placed this as an attachment to this document. With recognized text, we also are able to search documents in our system by the content of this document.
• Client way. The operator working with a personal scanner connected right to his machine. While our application web-based and the server is far from this scanner, we used only client resources to scan images. We used java-applet which worked directly with TWAIN sources, so operator just called some functionality right from our application after finishing the scan, it has appeared as an attachment in the document. But this case now has some issues due to limitations for JAVA plugins in modern browsers. But it is now possible to find some modern solutions which may help to get access to twain on modern browsers, you can just google it.

SSO, can be achieved in some different ways. It can work over OAuth2, NTLM, Kerberos, SAML and so on. In different projects, I have used Kerberos/NTLM and OAuth2. But real SSO was only with Kerberos. And when you already have LDAP Auth in your application, it will be quite easy to add SSO. But also depends on which OS and which WEB server you have. On Windows much easier to start with IIS while so difficult to find a working module for Apache. On Linux there is also could be a problem to find the latest version of the module which will work with the latest version of apache. But when you will manage to get it worked on web-server side, on Caché side, almost nothing to do left. When you get first unauthorized request, you should return back with status 401, and say which method of authentication you need through header WWW-Authenticate: NTLM. Then if web server managed to get username, it will send it by header REMOTE_USER. Of course, you will not get password, you just use this username and authorize session. 

If your server on windows, you have two connection options. 

• Terminal. Available only locally, and can use windows security. Enabled by default.
• Telnet. Used to connect from outside. Disabled by default (you can activate this service in SMP.). After enabling, you can connect using terminal or any other tool by default port 22.

If your server on Linux. You have only one option is csession or irissession tool which works only locally to the server. For remote access you should use ssh or telnet. But you can't configure telnet from Cache. You should do it by yourself.

CACHE.DAT or even IRIS.DAT does not have any relation with operation system. So, you can easily move database files between instances on different OS and versions. But you should remember about data itself, like differences between 8-bit and Unicode. And code, which should be compiled for particular version but also not depends on OS.

You can find some details about database format from my articles.

when you get the error <DYNAMIC LIBRARY LOAD>, you should look at cconsole.log (or messages.log for IRIS), where you may find code of error.

It is possible that you build it for 32bit, but uses in 64bit instance.  In this case, you will get the error with code 139. if you got other code, you can google it.

I'm sure it is quite difficult to write such a universal solution, every time it depends on your code. Years ago I wrote such a tool for an application which textual interface, and I had to convert parse code for pseudographics there and convert it too. So, I would say you need such specific too, just for your case.

STORE error means, your reached limit of memory per process. Since 2012.2 we have 256Mb per process by default and you can increase it up to 2Tb.  You can increase it, but I would recommend playing with different ways how to collect SQL Data, or on optimization in this SQL query. Or you can split such many rows by portions with less number of rows.