Well .. if the stems being eaten from the inside out and the root is gone - i'd say you're fighting a losing battle here to be honest. Like cres says, the first 2 weeks of an Autos life is extremely important and to be fair - with the scissors and bugs, the conditions haven't been ideal.
You could try and fix it with some recommendations from folks who are used to dealing with bugs and stuff - but my advice for what it's worth would be to take what you've learned and start over.
Sound harsh - but just think - the compost you have is questionable - right? and you've got your new light which should rock and you managed to bring the seedling up despite some early mistreatment ( I'm guessing your initial leaf wilted becuase either you watered it and left it wet, or perhaps it was laying on the wet soil - it's happened to me before).
Next time round, get some approved/recommended soil (biobizz lightmix will do the trick for you), and yeah, keep it in a tent.
Just my thought - i'm sure others will chime in soon enough ^_^