If your TV antenna does not require any power via the coax, then there's a chance that this *might* work. You would need to hear from someone who's an expert on DirecTV to make sure that the signal and power passing between the LNB (on the dish) do not interfere with the VHF/UHF OTA signals and vice-versa.
Remember that there are power (DC) and control signals (~22 kHz) passing up to the dish and downconverted satellite signals (~950 - ~1450 MHz) coming down from the dish. OTA TV signals occupy 54 - 806 MHz (now actually 54 - 698 MHz after the digital transition). In order for the diplexors to work, they must be able to combine/separate the signals at both ends of the cable and keep thing isolated enough such that the signals from one system do not cause problems for the other.
You need to be careful about where the DC power is going because you don't want to send power into your TV's RF input or any other places it's not supposed to go. If the power is going anywhere other than the dish itself, you might end up with insufficient voltage to power the dish. It's also unclear whether any of this will interfere with the 22 kHz control signals.
You might want to check around for independent confirmation from people that have successfully done this (not just from the people trying to sell the diplexors). As an RF engineer, I think the risk of cross-interference is high, but I would not go so far as to say it's impossible to make this work.
As for the antenna choice... You have stations coming from many different directions. If you go with a directional antenna like the one you pointed out, you will only be able to pick up one group of channels at a time, and you will need an antenna rotator to move the antenna around for the other channels.
The good thing about directional antennas is that you'll get more gain out of them, so you can pick up more channels down into the "red" zone on your list, assuming you've used an antenna rotator to aim it correctly.
If you go with an omni-directional antenna like the MS-1000 (non-amplified version of the MS-2002), you can pick up channels from multiple directions simultaneously, but the antenna gain won't be so good. With an omni antenna, you'll probably only get channels in the "green" and maybe some "yellow" zone channels.
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