Today we struggled with getting specific ProductFields into the "Cart Summary" view.
And we found out that the Extension Method GetProduct() on the CartOrderlineViewModel always return null. Maybe something is out of context the way the Express Buy App is used. Thinking about a form being posted and its using the Model.Cart from a PageViewModel?
Example og your GetProductLink that also doesn't work:
https://github.com/dynamicweb/Swift/blob/53596033e6b27234973b1a28131f1432e80e0506/Swift/Files/Templates/Designs/Swift/Swift_CartSummary.cshtml#L56
We made a workaround and added a custom call to ViewModelFactory to get our productViewModel.