1) I can't speak for Omni. But to me they seem to be cool with it. Spootnik is now running since two years and I asked them back then if they're fine with my integration. And they were. We're in contact from now and then and their updates have been pretty conservative so far. And as whpalmer4 already mentioned, you're free to decide when to upgrade to a new OmniFocus version.
2) Transparency and privacy is very important for me as without it would work. Your OmniFocus data sits on the same server as the Basecamp synchronization runs. All communication is secured by SSL (https with a trusted certificate). If you enable SSL communication on your Basecamp account the communication between Spootnik and Basecamp will be encrypted by SSL, too. The server is monitored by myself (exclusively) and only accessible over ssh (no password, just Public/Private keys) and http/https. All administration work happens over ssh using the command line (not a vulnerable web interface). Your data is only accessible with special access permission and I restrict myself not to access your data without your explicit permission. And I do everything to make sure no one else gets access to that data. See also http://www.spootnik.net/faq.html#basecamp_ssl
3) If you just want to synchronize your OmniFocus devices (iPhone, iPad, Mac) you can sign up for free. Go to http://www.spootnik.net/signup.html
and select "Free plan". Exclusive OmniFocus synchronization out of the box without the hassle to find a free WebDAV provider.
4) Yes, everyone who wants to synchronize its OmniFocus database with Basecamp needs a Spootnik account. Your Spootnik account can contain private stuff as well as business stuff. Usually the business stuff gets synchronized with Basecamp as that's the best way to collaborate with others. But not everyone at a company needs Spootnik. Those who are fine with Basecamp's front-end and don't want to collect all to-do items (e.g. private and business) in a single bucket won't need a Spootnik account.
Don't hesitate to write me (email@example.com) if you have any further questions.