Referee Magazine August 2003 Football Test Questions on page 19

[ABoselli]

Force is never a factor on kicks that go into R's end zone. The kick doesn't end until it is caught, goes OB or crosses the goal line. The ball is dead in the scenario you first submitted, so whether its K or R, it doesn't matter - it's dead, dead, dead.

In the original question, R recovered (picked up the grounded/muffed kick) therefore the kick was over at that point. He then, carried it into the end zone and was downed there - the momentum exception evaporated once the kick touched the ground.