A-C lengths are all over the place, so we test based on worst cases and limit the travel because otherwise it gets into all sorts of issues with ‘this fork is this long’ etc. Also, fork lengths are all quoted to us designers as being +-5mm, so your ‘short’ Fox fork is probably just as long as your ‘long’ Revelation once they’ve wondered around a bit. It’s also not just about the length, but the intent. Longer travel forks means faster riding means more load into the frame. And if we say 150 is OK, people start asking if they can just ‘push it out’ to 160mm.
140mm travel is a solid, easy to understand fork spec which we know 100% that the frame is durable for all the options.
As for the internal headset thing, you’re not supposed to put internal headsets in the bottom cup of our frames. You’ll have no adjuster clearance on the down tube and you are steepening everything up. Running a 150 travel fork with an internal headset will mean it’s steeper than a 140 or 130mm fork once it’s at ride height, but with more movement (and geometry change) and steeper at the extreme end of travel, so basically it’ll be not as nice to ride.
A 36 spaced to 140mm is around the same length as a 140mm Revelation or Magura fork. It will put more load into the frame than those forks, but given we lab test with a lab fork with solid steel bars for legs, I’m happy it will be OK. As I said above, it’s not just about the static length or the stiffness of the fork chassis, it’s about the intent that comes with extra travel.