So when you are doing a practice autoland do you brief the appropriate contingencies? Failures below 1000', alert height, no autoland warning etc? I certainly hope you do.
An autoland is not a Cat 1 approach. A practice autoland as mentioned is a Cat 3b flown within the met parameters of Cat 1 (invariably).
Some CAT I approaches require an autoland in this outfit (CAT I LTS), and at some operators CAT II/III approaches don't necessarily end in an autoland (aircraft fitted with HUD guidance, for example). You're getting confused with the terminology. If the manufacturer and your company allows it and the aircraft is appropriately certified, you can do an autoland off any type of approach you choose (although of course in practice I can't think of an aircraft type which can autoland off anything other than an ILS/MLS).
As to your first question: the whole idea is that we brief and fly the approach as a CATIIIb approach and autoland. Therefore, we tend to run through the standard low vis briefing points (task sharing, standard calls etc.) However, clearly go around decision making is different in the practice case to the real one. We tend to brief that if we get a loss of CATIII/ auto land warning light etc. and we can safely revert to CAT I, we'll continue the approach manually to avoid an unncecessary go around.
Provided you did request (to practice autoland) and have been cleared by ATC, you can keep your AP up to taxi speed (20kt if not mistaking)
Certainly on our route network (Europe) there is no requirement to tell ATC. It doesn't change anything for them, and therefore they're not interested. We used to do it, and would invariably be met by silence, then a confused 'Roger' and occasionally a 'LVP protections are not in place.' It may be different in other parts of the world of course.