OK, so just to clear some things up...
The 2018 MBA I just bought has 16GB of RAM. My old MBP 13 has 8GB and I've been running into that limit, but the fan rarely comes on...mostly I notice because I see the spinning circle and have to wait for it to figure things out. Then I start looking for things to close. But I think a part of my problem is that my 256GB storage is also close to its limit and my HD is probably extremely fragmented, so it can't swap to disk efficiently. If I were to totally reformat the drive, I'd bet it would start running pretty well again.
Also, I seriously doubt that this 2018 MBA was ever returned. It came double-boxed, and the outermost cardboard box (brown) had a strip-cardboard seal that I pulled to open that box up. That box had a sticker that had the model # and serial # on it, and looked to be from Apple. Inside that box was the white MBA Air box. It was shrink-wrapped, and upon opening it, everything looked brand new. The laptop was inside the translucent plastic sleeve. Removing it from that, the super-thin parchment paper was between the keyboard and screen. The power cable was tied with those white paper/cardboard things and were sealed, and everything looked completely new. The laptop had no marks, fingerprints, or otherwise. It would have taken a lot of effort for someone at B&H to repackage everything this meticulously. Lastly, I just checked my warranty coverage on Apple's website and it shows an expiration date of a year from yesterday (12/16/2020).
Back to my troubleshooting...last night after trying to reset the SMC several times, and not sure if I was accomplishing anything with that, I continued to try letting it charge for a while, then trying to boot up again. The problem continued: It boots up, and during the login step the fan starts cranking up. I should note that when going through the semi-lengthy initialization step, the fan was running the whole time but the case remained super-cold. Like not warm at all. This continued to be the situation as I tried charging for a while, booting up, using it, upgrading the OS (more on that in a sec), etc. The fan ran full-speed but the case never got remotely warm. So that also seems odd. Meanwhile the battery continues to show 0% with the "Condition: Replace Now" message. Also, at any time while it was booted up, if I unplugged my USB-C cable, the computer would *immediately* shut down. So, it's certainly acting like the battery is not holding *any* charge. Almost like there's no battery at all. More likely, the battery is there and may even be capable of being charged, but there's probably some defect that's causing it to think that the battery is useless and so it's not even *attempting* to recharge it at all.
So one of the Apple support pages suggested upgrading the OS. I don't see how this can help this problem, but I figured, why not. So I went through the process of downloading/upgrading the OS last night. I let it sit for a while, and after a long while it looked like it was still installing and possibly hung. The progress bar wasn't moving. Fan still running full-speed. So I powered it down and powered it back up again. During login, there's a progress bar that was moving slowly. Not sure if it was related to the OS upgrade, or what. That seems to hang as well. Powered it down and booted it up again. Same situation with the progress bar, but eventually I see the desktop and can use it. Again, fan running full-speed, 0% battery, and the computer is running super slow. Activity monitor doesn't seem to show anything getting a ton of activity, but the CPU gauge doesn't seem to be showing movement at all, so it's hard to tell what's going on. I forget to check the OS version to see if the upgrade had finalized (I don't think it has, because I see the same Mojave desktop wallpaper). That's when I also tried unplugging the USB-C cable and it shut down completely.
I then let it stay plugged in overnight and went to bed.
This morning, no improvement. I booted it up, click on my name to login, and the fan cranks up and I get a progress bar that within a couple of minutes seems to be hanging at around the 15% mark.
I guess I'll just have to chalk this one up as a lemon. I'm going to call B&H shortly and see what they're willing to do. I suspect I'll end up bringing it to my Apple store (about 30 mins away) and hoping that turnaround time isn't horrible. Fortunately, I still have my working 2013 MBP 13 and hadn't gotten to the point of even installing any software on the "new" 2018 MBA.