Choosing right mesh count for high-detail, neon plastisol print

You've got the right idea with underbasing it and a top coat, both with 230 screens.
If your screens are tight and your press holds good registration, then you shouldn't have a problem choking the underbase by only a little bit to make sure you hold the smaller lines in the design.

If your design is vector you can convert to raster and choke by a few pixels. Yes the under base may may loose a little bits here and there but if its vector and you choke by a pt. you will loose a lot.

I would under base just as you thinking. With a 230 a reduced white. Then over print with a neon green. You can try some hi opacity neon green just print flask print and skip the under base and that's fine for some but I don't care what anyone says the ink will ALWAYS be brighter with the white base even a thin, reduced white base and using a high mesh will also help keep the hand down. I would over print with either a 200 or a 260-280. Just don't use the same as the underbase as you can sometime see a pattern if you use the same screen for both.