I already answered this for you. You have to "download" the notebooks you want to use off-line. Once you do the next time you open the app when not connected, it will open and the notebooks you downloaded will be available. I have tested this and am 100% certain of my answer.
You really needn't worry about Evernote dying out or going away. While there is no guarantee, Evernote is a company that is growing and is in it for the long haul. But if that does happen I'm sure they will give us all plenty of notice so we have the time to download/export all our notes.
As for Omnifocus, yes it is an awesome application. Why do you feel you need the Windows version to make this thing work?
When you are using a Windows PC you can simply email a task to your own Omnifocus sync email address. Or you could add tasks to the iPod touch app and sync it when you get home (or do you have an iPhone?). I'm assuming your Mac is your home computer and the Windows PC is at work? If that is the case, do you really need a full blown app at work? Wouldn't the iPod touch/iPhone and the ability to email tasks to Omnifocus suffice?
If you still feel that you need an all out full version app on Mac, Windows and iPod/iPhone then start using Evernote and get busy. You can get stuck by information paralysis. At this point you have all the info you need. Download Evernote and start setting it up