Except for one problem: the SPICE is only found on Arrakis. If we are using the pre-Heretics of Dune books as the guiding line, particularly during Dune and Dune Messiah (books 1&2), then spice is only on Arrakis.
Between books 4 and 5 the Tleilaxu (uber-geneticists) found a way to synthesize the spice. In one of the 2 books that came after #6 (Chapterhouse:Dune), there was a succesful transplanet of a water-based worm to the planet "Buzzell".
If we use pre-book-3 (Children of Dune) where Paul is still in power (no he was not overthrown), then the spice is ONLY on Arrakis. And worms CANNOT be transplanted to another planet (like say, Tatooine). This is because, of the simple fact that Arrakis ESSENTIALLY HAS NO WATER. It just doesn't have any natural, "easily" accessible wells. Wells that ARE dug and used quickly seal up.
Then, there is the fact that on Arrakis, conditions are FAR more extreme than on Tatooine. If the Empire wants to use the spice as a resource for subjugation (now you're addicted, and I control the drug, so you must obey me!), then they won't bomb the surface. The Imperial Stormtroopers will probably land way out in the desert. Then, the pattern of: "If the Coriolis Storms don't get them, the worms will. If the worms don't get them, the Fremen will. If the Fremen don't get them, then they will die of dehydration." sets in.
I've always thought of the guild as pretty cowardly, so I always assumed it was their lives, then the spice. Not to mention if they refuse to help the freman the Empire can pick off the stranded freman at their leasure. After the Freman are gone, the guild will have easier access to spice than ever before.
As for your second arguement, why do you assume the Empire cares about Arrakis anyways? They have their own spice on Kessel, not the same as Arrakis, but potent enough, and even better they don't really need Arrakian spice for anything. Thus, whats to stop them from just destroying the planet? I do agree that the Empire will lose in a ground battle, but that is not the Empire's best force. Once they realize their ground forces are getting decimated, they'll just destroy the planet from space.