Maybe I've had it wrong, but my understanding was that committing had more to do with wx deteriorating after your PNR. ie destination alternate suddenly required prior to PNR - continue if you wish to PNR then to your alternate, however as soon as you pass your PNR you are "committed" to your destination, regardless of the wx. Simply comes down to fuel in the tanks.
Without going too far off topic, is there any regulatory requirement to have the latest TTF?
eg. Last TTF received is valid for your arrival and just above alternate minima. Even though it doesn't say so, your experience suggests it may deteriorate below alternate, but probably not below landing minima. Airmanship aside, are you required to obtain the next TTF which may have alternate requirements?