Is there some official source to confirm which date the account is charged (that is, at what point I will no longer be able to use the fifty dollars which, prior to that point, was in my checking account).
The way that the system works is that you need a serial key registered in order to download. That's how people with it (be it preorder or final) are distinguished from those without. If you have an uncharged preorder, it's not even linked to your Impulse account in any way yet, it's just queued up in sales.
When the beta is opened, all pending preordeers are charged through and those people are issued their serials so they can have access. Those steps (buy, get/register serial, granted access) are inseparable because there's no differentiation between a beta key and a final key.
If you do not wish to tie up your money at such an early stage or want to try the game before doing so, I'd advise against preordering. We usually do a demo after final if that's more your thing, and you can still propose ideas and such on the forum based on others' comments without having the beta yourself.
The current plan is for the beta to open sometime next weekend or shortly following (during/after PAX).