Well, someone already mentioned to bypass all of the ignition system, by jumpering the battery directly to the FBO ECU. So if you can't get within 1/2 volt of battery voltage there during cranking using this method, then disable the ignition system, charge the battery, and try again. If the battery voltage is still low, then take out the plugs, charge the battery,and try again. There has to be a legitimate reason that the battery is falling to 9.5, and since you already subbed in a different battery, and a different starter, then it sorta has to be mechanical; like liquid in the cylinders,over-advanced ignition, or dragging internal parts, just to name the most likely.
