Yes, well, save for the "fleshy lips" thing. They're just labile, but passive structures: there were no muscles and they wouldn't lift to "snarl" or anything.
So, I assume, if they were not expressive, these "lips" would presumably just have been to aid in food retention?
Not really. These structures do not serve that function in extant reptiles. Instead, they tend to be more about sensation and protection of the oral tissues, to retard dehydration in animals that dwell in arid regions, but also yes, to "retain" some types of food. Note that one of the analogues to these tissues are the number, size and extent of the foramina, which indicate the size of branches of blood vessels (= thickness of tissues requiring nutrition) and nerves (= more sensation). Food retention doesn't seem to be much if any of a problem: You still see cattle and humans spilling food, despite their huge cheeks, and animals like geese and iguanas who lack them able to function fine.
true. Maybe "sensing food when biting" would be a better word.
Sensation around the oral margin is the best explanation, but not only when biting: When drinking, when "nuzzling," when grooming (even if they had beaks!), etc.
cool 