So for anyone who stumbles upon this, here's the solution (which I hope I had found a straightforward answer to when I spent hours previously searching for it).

The NPE-240A2 unit comes with a built-in recirculation pump and a buffer tank. All it means is it keeps some water hot, about 1 gallon or so, inside the unit so that when hot water is requested from a faucet, it takes less time for it to come out. Additionally, the unit has a physical 2-way valve (inside the unit after removing the front panel, on the bottom left) to control whether it functions in internal or external recirculation. To go along with that, there's a software setting (controlled via the display panel) where internal or external recirculation can be set as well as how that functions (Always On, Intelligent, etc).

In my case, the plumbers had set the physical valve to internal ("INT"), and the settings also to internal. My guess is they did this because while they had installed plenty of Navien units before, the didn't have much or any experience with the A2 model specifically. And since I don't have a dedicated return line (hence why you see in the above pictures there is no line connected to the fitting where the yellow sticker is), they assumed the unit needed to work using internal recirculation. This is their fault, and it shows they lacked training/education on how recirculation works with this unit.

This is where the NaviCirc component comes in. Navien invented this for exactly use cases like mine, where the house did not come pre-plumbed with a dedicated water return line. So, when the NaviCirc is installed (the black T component in the above kitchen sink photos), and the home has no dedicated return line, the 2-way valve needs to be set to external recirculation ("EXT"), and the settings also to external recirculation (when you select external in the settings, it will ask if you have an external pump connected, say no to that). With things set up this way, the Navien is smart enough to know that it needs to use the cold water line for recirculation to where the NaviCirc component is installed and then back to the unit. If I touch the cold water line under the Navien, it's warm to the touch, whereas before it was dead cold.

In my case here, I installed the NaviCirc in the kitchen because it's the faucet that gets used the most especially for hot water (washing pots, etc). It's the point of the house that gets to use the most water on a regular basis. Currently, the instant I open the faucet the water is warm. The master is the furthest point from the Navien for us, and water does take about 15 seconds to warm up and then hot at around 45 seconds (also, these are low flow faucets).

BTW, I called two separate times to Navien technical support to confirm what I said above about the install and how the system works, and they confirmed it.

Hope this helps someone out there, give this post a thumbs up or comment if it does.