In reviewing https://wyday.com/limelm/help/using-turboactivate-with-csharp/, I see that our app should be calling IsGenuine every 90 days. I assume this would be an issue for customers using our software on closed networks, as this call would always throw an InternetException?
That's correct. For offline-activated users you can handle this a few ways. The easiest thing you can do is to not worry about IsGenuine() calls for these users because they will be such a small part of your target audience (much less than 1% if you're like our other customers).
Or you can offline activate again. That is, have the customer send the activation request file again.
Does that make sense?