Negative attributes have never been a thing in Shadowrun to my knowledge. Once it hits 0 it stays there AFAIK, just waiting for 5 Karma.
And per the discussion I had in the GM forms where I argued you should do steps in order, I was told I was wrong. So, here we are. Numbered steps doesn't mean you do them in order, it's a wacky world.
The "Do things in order" was an argument in 5th edition too, as was the hit 0 and bounce back during chargen, and Used Cultured Bioware. You've got several threads from .... 2014? was it? that you can recycle the arguments from. Pick a side and bring up some Necro-threads if you'd like, I'm in a few of them and have no interest in replaying them.
I personally welcome Errata or Missions FAQ that clean any of that up either way. Until then, I roll with what I've got on the .PDF.