if your corpus luteum is not making enough progesterone to keep you preg, and we are saying it's not here because you are spotting (and spotting usually means your progesterone is between 4-9, you want it to be at least 10, 15 or higher is better,) stopping provera on cd28 will cause withdrawal bleeding, whether you are pregnant or not, because you have essentially no progesterone of your own on board. (i would say there is a small chance when you get pg the hcg may rescue even a wimpy c.l. and make enough progesterone on it's own, but that is not a bet i would take.)
the way prometrium (or crinone) is prescribed is, you take it from o (or 3 days post o, when temps verify that you did in fact ovulate) until 14 dpo, and if you have a bfn, you stop it, get your af, and start a new cycle. (although sometimes you wont make it that far, it depends on how you respond to the progesterone.)
if you get a bfp by 14 dpo, you continue on the progesterone through (usually) 12 wks, when the placenta has grown big enough to support the pregnancy (this probably really happens sooner than this, but this is the standard recommendation.)
i would really really have concerns about this plan if i were you-