I think you are overthinking it. By new accounts they usually mean that you haven't been a t-mobile postpaid customer for 30-90 days or so.
Just because your balance goes to zero for a few days doesn't mean they won't let you back on. Not sure how long after your balance goes to 0 you lose your number though.
ok, thats how i thought it was. only reason i worry is because in the past i accidentally let my sprint account go 3 months without paying (new debit card, and sprint didnt have my email or anything to notify me)
once i realized it, i called sprint and told them "i dont wanna let my plan get taken from me!" and the lady laughed, saying they werent even thinking about it. she told me that sprint almost never kills peoples chances to come back... which made sense to me in their situation.
but like you said, ill know the first day if i miss the payments. i wont be able to use my phone