1. To return to any OFW, use a downgrade "jig" and the 3.55 files.
2. is Sony able to prove any CFW installed before? If the latest news is correct, YES from their PSN logs. However, if you were to downgrade to even modified 3.41, then format the hard drive and update to 3.55 via USB media, THEN connect to PSN and update to 3.56 or whatever the latest is at that point, no. I'd say it's highly unlikely they could look at that machine and say that it has a CFW or MFW on it at any point just from looking at it.
3. Sony will likely send you a machine back with the latest firmware on it, especially in this "climate". RIGHT NOW there is no hack for 3.56, but things are changing every day.