One more thing, can I register domains off line? What are the pros and cons of that?
So you mean that search engines are also "blind" to private registrations?
The way it usually works (afaik) is that the registrar registers it using their own name and address details but on your behalf. Like a mail forwarding service.
So it's not a matter of them being 'blind' or not to the registration details - the registration details simply don't contain that info.
My concern is that google does not allow one company to have multiple high ranking sites or does it?
No, that's totally false. They do. Try Googling 'fox' (or 'disney' or 'microsoft') to see lots of high-ranking sites belonging to one company.
What they don't allow is for you to have a bunch of different sites with exactly the same content. Well, they can't stop you from owning the sites, but it'll hurt your rankings.
hth, a.