I've tried using the search function, but I'm not quite finding what I'm looking for. I recently finished up my FC1 with no waivers needed and am waiting for the approval to come back from AETC before my package is sent to State HQ and NGB. At this point, is it truly just a waiting game to have all the boxes checked off, or would there still be for any reason my package would not be approved? The reason for asking is I'm getting into a bind at my civilian job where my potential replacement could be transferring out of the region within the next month, and my supervisor and I would really like to keep him here to be able to back fill my position once I'm off to OTS, UPT, etc. Otherwise, I'd rather not tell the higher-ups until I have dates, but my hand may be forced very shortly.

Check the Guard/Reserve Timeline to UPT thread. It's typically 4-6 weeks after FC1/MFS you'll receive paperwork stamped approved. Then, expect another 4-8 weeks for NGB packages to make its way through the loop. For me, OTS dates were scheduled about 4 months out after hearing back from NGB. So yes, it is all a waiting game. YMMV. 

My apologies if my question wasn't worded clearly, but I'm just wanting to know if there's any reason where my package would not be approved even though I've passed all physicals and have made it through the preliminary paperwork.

In my opinion, as long as you meet all the minimum requirements and don't have any waivers that need approval/disapproval then you should be good to go (after the waiting game of course). Your recruiter should be able to give you a more complete answer as they have seen your package. 

Stay on top of your recruiter. My package took a long time because it was being sent up incorrect multiple times.