There are a lot of variables here, but my impression is that you should be fine without charging in the middle of a road trip. When you're not charging, you're driving it like an HEV instead of a PHEV and it still tries to maintain about a 20% charge (which equates to approximately a 0 mile EV range in the PHEV).
Your husband is right about the battery needing to be maintained above zero percent charge, but it sounds like he's not sufficiently informed about the Niro: The PHEV seems to map 0 miles range to about 20% charge (you can see this on the head unit if you play around with the PHEV application). So it's not likely to ever get to 0% charge in most normal driving situations. The computer tries to protect the battery from falling into that condition.
Charging with an extension cord is potentially a bad idea. The majority of extension cords are light-gauge wire (14 gauge or higher/thinner), which means that they're not necessarily designed to carry the high current that even the L1 charger requires. When charging for a period of time (more than 15 minutes for example), you might even notice that the extension cord feels warm to the touch. If so, that tells you that at best you're wasting electricity, and at worst you're potentially engaging in a charging exercise that could lead to circuit failure or a house fire. Somewhere in the middle of that spectrum you might also wonder if the voltage drop from using a light-gauge cord might be impacting your charging electronics in an adverse way. If you really want to charge with an extension cord, get a thick (12 gauge or even 10 gauge if you can find it) heavy duty cord, the shorter the better (25 ft or less). You could also dial back your Niro's programmed charging rate to low and probably get by with a thinner (14 gauge) cord for occasional use, although I don't think Kia or anyone else would ever "bless" that approach.
But really, that's the sweet thing about the PHEV is that you don't need to charge it on a road trip. It does a pretty good job of taking care of everything automatically. All you have to do is put gas in it before it hits empty.
The most simplistic way to drive your PHEV is simply to pull out of the driveway and go as far as you can go on your all-electric range, and then rely on the car to automatically switch into HEV mode for the duration of the trip.
A somewhat smarter way is to pull out of your driveway and and burn off about 25% of your electric range (so get it down to maybe 18 miles of EV range) and then, depending on your driving conditions, maybe push the button next to the gear shift to switch into HEV mode. If you do that, don't be surprised if the ICE doesn't start for a minute or two, depending on your driving conditions. But it will eventually start, and it will try to maintain the EV range that you had at the moment that you toggled it into HEV mode. (Except that every time you shut it off and restart it, it will start in EV mode and try to use up your remaining EV range unless you remember to toggle it again).
You want to burn off some of your 26 mile charge so that you can take advantage of regenerative braking (so that you have capacity to absorb charge without overcharging the battery). I don't know what's optimal, but 25% is what I use.
If you've got something like 18 miles of EV range, and you're driving in HEV mode, the computer will take advantage of the surplus charge in the battery if it needs it (going up a mountain road for example) but it will try to restore it to 18 miles when it finds an opportunity to do that.
If you find yourself in stop and go traffic for a few miles, you can manually switch back to EV mode and avoid the constant start/stop of the ICE, which should benefit your overall fuel economy and also reduce wear and tear on your ICE/starter.
When you're coming home from your road trip, you can enjoy maximum fuel economy for the overall trip by switching back to EV mode when you are about as many miles from your home charger as indicated by your EV range. In other words, use up the remaining75% of your range at the end of the trip, rather than at the outset (in practice, if you try to follow this guidance, you're like to have less than 75% left at the end, because you probably used some for stop-and-go traffic along the way).
2018 Niro PHEV Gravity Blue