As far as using predatory nematodes, I have used some in my garden to deal with japanese beetle larva and other vegetable garden invaders. But as far as results are concerned, it is difficult to say. Predatory nematodes is like releasing a couple cats into the neighborhood & hoping they solve a mouse problem! Some will indeed help out, however it won't be as effective as putting out poison or traps. (Or time to call the Pied Piper!)
Most natural remedies are like that (IME) which is why the other methods were invented in the first place. (They do obviously have their drawbacks however.) Not that predatory nematodes can't help, but I wouldn't expect miracles from them. At least that's my opinion & experience, and others can certainly have great results that I haven't.
I do use Trichoderma & other natural means for doing things, so don't think that my mind isn't open to other things. But when it comes to things like what you are experiencing with your plants, I tend to find that the majority of what I was taught years ago still holds true, & the old methods are many times still the best way.
As far as the car worm meds, I would certainly go for it! First see if they kill the beasties in a separate container, then if that does kill them off, take a sacrificial VFT & give it a try to see how it does. That us exactly how new methods and ways are discovered, innovation! (I am actually a big proponent of that!)
Well, again good luck & do keep me posted on how it proceeds! I do wonder where these things came to you in the first place, & I'd watch to make sure it doesn't spread. I've caught various "bugs" from trades here on TF (no names mentioned), and it can be a real pain to eradicate them. But that is part of becoming a good grower, and really is good experience. You can't claim to be great at growing plants if you never have to deal with difficult situations. So in a way I congratulate you & want you to know you are not alone in this.
If worse comes to worse, we can always get you some new plants.