No NSF != Guaranteed funds
Any payor has the right to dispute a charge. That is a Federal Regulation (Reg E ) so no payment system can guarantee funds (other than wire transfers which are fund transfers not transactions i.e. not funds paid for goods/services received )
We guarantee no NSFs. With ACH NSFs are a big issue since until the 2-3 settlement is done one does not know whether the funds are going to clear. What the Payor disputes with the Merchant is between Payor and the Merchant
In so far as SS# collection etc. it applies only to the receiver not to the sender. MTA laws apply only if funds are being held.
As far as guest checkout for Dwolla and other services - please try it and let me know how far you can get without signing up for the service... the way I see it is guest checkout is simply deferring sign-up not obviating it.