Is there a somewhat simple way for an admin user to 'log into' or view a user's (doesn't have to be an account holding user) shopping cart?
It would be nice even to move through the checkout progress for them as well, but just the cart would at least be a good first step?
You can manage user carts in the Admin Panel -> Orders. To see incomplete Orders click Filters and uncheck show only complete orders. Then you can access Order page and edit their Cart contents, shipments, payments and so on.
See https://guides.spreecommerce.org/user/editing_orders.html