I can understand your frustration, I was in a similar situation. I got passed over the first time (and could never find out why, had all the check boxes I needed), and after I got passed over I got PCS'd with training enroute to my next duty station. When I got to my next duty station, I was there 2-3 months before the board met. So the only way you can get promoted after being passed over once is if you get a DP the second time around, which I obviously did not get having never met the WG/CC at my new base.
So, to answer your question, if you get passed over twice you most likely will get offered continuation. If you decline continuation it wipes out any and all commitments you may have from PCS, ADSC, etc. Once you decline, you have to separate in 180 days or less. Guard or reserve is an option, but you will have to get a waiver to get in for being passed over twice, and it is up to the unit if they want to pursue that for you. Good luck, as it is a crummy situation which I ended up in too, but much happier living in civilian life where I control what I do.