Barcode detection definitely does not depend on TWAIN except in rare cases.
The catch with barcode detection is that some TWAIN scanners (or their drivers) can do it, but most can’t.
So – if you know that all your scanners can do this, then you can have the scanner do the work. However, figuring out how to do that is quite complicated, you’ll need to study and understand the Extended Image Information feature of TWAIN, for example.
If you are writing an application that needs to work with scanners that don’t do barcode recognition (i.e. most scanners) then it has to be done in software, as part of your application. Many imaging toolkits can do barcode recognition.
You can also license a barcode-recognition library, there are a lot of them (Inlite, OnBarcode, many others). And there are some free barcode recognition libraries, such as Zebra Crossing (https://github.com/zxing/zxing)
It’s more common for TWAIN scanners to support automatic blank-page discarding, but certainly not universal – definitely not flatbeds, and I wouldn’t count on most multi-function devices doing it either. Almost any imaging library should be able to do blank-page detection although you may have to do some digging to figure out how to code it.