A few things about the examples above.
On Earth's rotation staying relatively constant - yes, it does stay relatively unchanged, except if it falls under the gravitational influence of another celestial body. We have evidence of collisions of celestial bodies (the Solar System's asteroid belt), so it is theoretically possible that Earth's orbit and rotation cycles to be influenced by a large enough body, which would considerably change the length of Earth's day. Using this as analogy, the mana cycle might have been influenced by an extraordinary event, which has accelerated the rate of mana growth, thus nearing the time of Horrors. I'd say this is the implication ever since Shadowrun 3e (when Shadowrun and Earthdawn were both released by the same publisher).
On the threshold's importance - the best analogy I can think of is a water dam. When no water passes the dam wall, you have no water on the other side, so the landscape is much more different. When water passes the top of the wall, however, you start getting water, which nourishes plants, then trees, new life, etc., namely a lot of things you wouldn't have without water. Same thing with mana - passing a certain threshold, it allows expression of metahuman traits, allows magic, and an increasingly potent one. There is a good real-life example with the Colorado River, which by the time it reaches Mexico is an insignificant fraction of what it is in the US, mostly due to heavy water consumption. Several years ago, US authorities decided to let the water through, I don't remember the exact reason. The Mexican landscape around the river changed in days, with people seeing green for the first time in decades.
So basically this is it - once mana reaches necessary levels, it allows certain things to happen. Now, there appear to be a lot of things wrong with this cycle, since goblinisation wasn't supposed to happen so early, as implied in Forbidden Arcana. Furthermore, some Earthdawn races are still absent, so there may be a while lot of things going on ...