1) There is an unclassified signature program for self-defined class of
drivers. Read this page for more information
2) You cannot get logo (according to the article) if you define your own
class. You define your own class if your device functionality doesn't fit
any of the microsoft defined classes.
3) Every PNP enumerated device will have information under Enum key. Don't
mess with that.
4) No it wouldn't. Setup class GUID in the INF means something to the
system. For network class, the system manages the binding of your device
with lot of other components in the system. So if you change the GUID, your
device wouldn't work as a network device.