I opened my R/B after kid dropped Switch down stairs; some time later I had no issues with an exchange.

No. Opening a device is categorically NOT a warranty voiding action, by law. At least in the US. Though, in this case, it does sound like they damaged the joycon, which would make it not eligible for warranty, but it has nothing to do with opening the device.

Yeah it's got no signs of life, even when connected directly. I opened it to replace the plastic clips that lock the joycons onto the tablet, as after only a few months they had worn down to the point of being useless. I could have sent it to Nintendo and that would have probably been the smarter option, but I noticed IFixit sold metal replacements and the job seemed simple enough. I did the replacement on both joycons and right after they both worked fine. It was only the next morning that the purple one ceased function. My red/scarlet one still works fine.