Does the Trex keep eating 3x the fat after minute 60 if we keep going?

Maximum fat burning.

Yes. However maximum fat burn is during minute 21 and minute 60. If want a little extra fat burn it is fine to increase a little as you will still burn fat by adding an extra 15 minutes. The program is very efficient, so best to concentrate on completing as close to design as possible. See page 24 in the Master Plan.