Electronic – arduino – Should I shield digital sensors in close proximity to one another? If so, how

It sounds like you are correct about the low side NPN and pullup resistor inside the sensor unit. You can make the output any lower voltage you want with the addition of a single resistor:

The resistor inside the sensor (R1) and a resistor you add (R2) form a voltage divider. The output high level will be V+ attenuated by the voltage divider:

\$ V_{OUT} = V_+ \times \dfrac{R2}{R1+R2} \$

In the example shown, the output will be 5V when high. The impedance of the OUT line will be the parallel combination of R1 and R2, which is 5 kΩ in this example.

You could also replace R2 with a 5 V zener if V+ might vary. However, my experience with sensors in general unless they explicitly say otherwise is that they can be surprisingly susceptible to power variations. I would at least filter V+, preferably regulate it.

The Patch-Plane Antenna

The antenna in question is is called a patch-plane (or plane-patch) antenna -- a patch antenna with a reference plane behind it. You can see it here (the four lighter green squares near the corners).

According to the datasheet, the directivity looks like this:

From behind its detection capability is about 63 times weaker assuming you are on the same level with it. If you are not on the same level, it is more than 250 times weaker.

So... perhaps you mount it up high and angle it downward. This will also help you to control detection range.

You should also be aware of and try to avoid reflections. RF is just EM (like light) and it bounces off stuff too! ...especially metals and liquid electrolytic solutions (like salt water).

The Reflector

A wave-guide won't help. What you need here is a reflector. I'm sure you've seen a satellite dish before. That's it's purpose: to refocus the back lobe into the forward direction and reduce the side lobes (e.g. improve directivity).

Designing one is beyond the scope of what I will answer here. If you choose to proceed with a reflector approach, ask again with your more specific questions, @reference me, and I'll respond in more detail.

Initial Search Volume Approach

Actually, I invented a RADAR architecture specifically designed to deal with this type of scenario called Angle-of-Arrival-Assisted Relative Interferometry (ARI) and wrote a few academic papers on the topic ([A], [B], [C], etc). A complete implementation is probably beyond you (and overkill for your application), but here's the basic idea:

You can partially overlap multiple modules and correlate their detections to reject out-of-area-of-interest targets. You can create a nice focused detection zone in this way. In ARI, that intersection region is called the Initial Search Volume (ISV).

In Conclusion...

Good luck! Neat project.

Quick note (thanks @AndrejaKo!):

Do note that if someone uses @ to reference you in a question in which you haven't posted before, you won't receive notification. Better option would be to place the reference here or to use the @ in chat.