1. IF the firms do OCI, my vote would be no to contacting early; they'll just push you off anyway until OCI. You would be better served by focusing early efforts on other avenues (e.g. firms that aren't doing your school's OCI).
2. I don't know about chances, but they absolutely do not look at first semester 2L grades. Your interviews and call backs will mostly be in EARLY fall, long before you're even thinking about exams. The 2L OCI hiring process is mostly completed by mid-November, before 1L recruiting (such as it is) starts Dec 1.
3. Yes, see above for timing. Don't waste your time re-applying to firms that rejected you during OCI. By the time the process is over, firms have more than enough applicants they'd like to fill spaces with. And unless their first, second, third choices all turn the firm down, they won't be considering new applications from 2Ls.
4. You can try to apply for 3L OCI or send a app in, but it will be an uphill battle. Unless more 2Ls than expected decide not to accept permanent offers and go elsewhere OR if more than expected decide to clerk after law school before work, a firm probably won't have any open new associate spaces. At that point, you'd be looking at trying to lateral in after a few years.
As for whether you're screwed, I don't know. You may get an offer to come back from your job this summer, which may help your recruiting chances a lot via OCI. But there are plenty of jobs out there that don't come on a silver platter via OCI. Get Kimm Walton's Guerrilla Tactics for Getting the Legal Job of Your Dreams, start networking in the area you want to be in, and research midsized and smaller firms that are located in the area you want to work. There are a ton of great jobs out there, but you seem to have convinced yourself that OCI = the ONLY jobs available.