Welcome, rockin1jr:
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A) I’ve read a few different places about the spacing, for identical stacked antennas. 3 feet, or 25 inches. What is the ideal spacing?
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For horizontal stacking, Calaveras knows more about it than anyone I know. I suggest you go to AVS and get his attention.
http://www.avsforum.com/forum/25-hdtv-technical/
His 91XGs looks like this:
http://www.aa6g.org/DTV/index.html
and his antenna system diagram is here:
http://www.aa6g.org/DTV/ABD/Antenna_Block_Diagram.html
I suggest you try just one 91XG at first for a comparison with your HDB8X. One 91XG has a narrow horizontal beamwidth, so azimuth aim is critical. For best results, the antenna must be aimed directly at the transmitter. You will not be able to "split the difference."
The next step up would be to stack two 91XGs. With two 91XGs, aim is even more critical. You can use a splitter in reverse as a combiner. The two coax lines must be the same length.
To keep losses low, Calaveras is using halfwave coaxial baluns, and combining the two antennas in parallel, giving 37.5 ohms, which is converted to 75 ohms by a quarter wave 50 ohm matching section. Any losses before the input of the preamp directly reduce the antenna gain. Losses after the preamp are less harmful because of the Friis Cascaded Noise Figure Equation that says the preamp NF primarily determines the system noise figure.
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B) What are my chances of getting the signals with the lowest noise margins? I’ve done my research about needing a NM above 0 to get a channel, or above 5 for a solid picture in all conditions.
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The 0 dB NM on the tvfool report is based upon a dipole antenna (0 dBd). You can add antenna gain and preamp gain to make the NM more positive, but you must subtract the preamp NF.
For reliable reception you should add a "fade margin" of 10 dB because OTA signals constantly vary in strength, but I doubt that you will have that luxury with the very weak signals you want.