Thanks for the responses.
Yes, I have the free roadside assistance, although I vaguely recall one or two people commenting on this forum that trying to use it was a painful experience (something about a recorded message asking the owner to type in the VIN, and no keyboard to type with). And I'm not really expecting to have an out-of-battery-charge condition, but there are a few threads on this forum where others did experience that. The manual notes that the vanity lights over the visors and the interior light that comes on when the back hatch is opened will not automatically shut off and can wear down the battery. I've already observed on two occasions that it's possible to get the back hatch in a half-latched state where you can't push it in to get it fully latched, but you also can't open it from the exterior button on the door. I don't know if the interior light comes on in that scenario, but I imagine it could. When this has come up for me, the only thing that worked was to use the key fob button that unlatches the hatch.
A few people have noted that the trickle charger that comes with the PHEV is described in the manual as an "emergency charger". The manual also recommends keeping it in the car. I'm starting to think that it is recommended to keep it in the car in case you find yourself with a flat traction battery for some unexpected reason, because booster cables probably won't solve the problem if both batteries are flat.
It looks like my electric range goes to zero when the high voltage battery has about 20% capacity, but I've seen the battery get as low as 6% (as reported by the PHEV app on the dashboard). In that condition, if someone were to leave a visor light on for a day or two and the "Battery Saver" were to run multiple times to replenish the 12V battery from what's left of the high voltage battery, it's easy to imagine that both could be low enough to prevent starting the ICE.
As for giving someone else a boost, I wonder if temporarily disabling the PHEV's "Battery Saver+" feature, and hooking up the cables with the ignition turned off, would be safe for both cars? In other threads, certain HEV owners have commented that they plan to decline to ever give anyone a boost. That's easy to say, until it's your wife or your brother that needs a boost, and maybe there are urgent circumstances, and your Niro is the only option around...
It would be nice if the Kia owner's manual documentation was clearer on some of these questions.
2018 Niro PHEV Gravity Blue