Will there be private rooms available or do we need to plan on sharing?
You can rent a private room, of course. The rooms are suites - one-bedroom, two-bedroom, and three-bedroom. The multi-bedroom suites have two bathrooms. Sharing is merely an option for those looking to reduce cost, since the rooms are rather pricy if you're not sharing.
Staying at the headquarters hotel puts you in the middle of all the action, and is generally preferable to staying some distance away and driving back and forth.
Also, if I call now and mention the NSXPO, will they know what I am talking about?
Yes.
Actually, I still haven't decided if I will drive my NSX up there at all. I don't plan on putting it on the track, so, if it's just going to sit in the hotel parking lot, I may just leave it at home.
For those driving to the event, it's really a kick to drive up there in a whole group of NSX's, and to have your car in the group photo, and to drive your NSX with all the other NSX's up to Oshkosh, etc. For those flying long distances, having their car there is not much of an option, but I think most folks would prefer to have their own NSX there if it can be arranged. Your choice, of course.
I don't know anything about the Osthoff but we always stayed at Siebkens when I was in IMSA racing.
The Osthoff just opened a few years ago, and is already considered "the" place to stay in the area. It includes a bunch of time-share condominium units that are owned by some of the most famous names in racing.
Siebken's is a not-all-that-nice place with on-street parking only. If the price of the Osthoff is a problem, there are other choices in Sheboygan (13 miles away) and Plymouth (6 miles away) that would be preferable to Siebken's IMO.