Stability/instability aren't really relevant there. In fact, this isn't even an issue of a strong non-linearity, it's just an issue of running into an actuation limit on the elevator control surface. The trim basically gives the elevators a boost.
Great theory, but here are the holes. If MCAS basically gives the elevators a necessary boost, you're suggesting full elevator down wouldn't even prevent the plane from runaway nose up stall when in a condition where the NG would be considered to still be in a normal part of its envelope. Where the NG would still be stable, and by your definition, stable means the NG would return towards straight even if you let go of the stick, let alone push the stick forward. So the MCAS has to kick in and help the pilot do something he wouldn't even do. If he thinks he can just let go of the stick, and the plane will be stable, why would he push full forward?
By your reasoning:
1. Ideally, the MCAS would not even be needed. They would make the elevator larger and more powerful. They could even limit the upwards range a bit, to ensure that upwards elevator is not increased. Only the downwards effect is increased. Of course this might not be possible without major alterations and cost and time and recertification, and perhaps due to these reasons it would be better to just build a new plane from scratch, even though it sounds simple.
2. If the MCAS is only supposed to boost or extend the pilot's response, then it would ideally only activate when the pilot is already at full elevator down. MCAS would not do anything if the pilot is not at full elevator down. In the case of these crashes, the pilots were obviously giving full elevator UP. But maybe trim control is too slow for this to work and/or maybe as I suggested earlier, an NG pilot wouldn't even know to push the stick that far forward until it was too late.
Any way you slice it, it would appear that MCAS is potentially a pathetic bandaid on a pretty serious wound.
So no. I think non-linearity and instability are potentially a big part of the problem. If Boeing were willing to eat it and redesign/recertify a new airframe, they would have made the plane with taller landing gear so the engines did not have to mount in a way that produced instability in a given AOA range where the plane still has laminar flow and lift. To me, "unstable" potentially means that even if the pilot had the range of control (with help of MCAS or otherwise) to reign the plane back from here, this area of AOA would be difficult to control and essentially unsafe/unusable without some active electronic aid that can respond relatively quickly and dynamically... not just to slam the nose back down, but to enable the pilot to utilize and fly in this range of AOA in a predictable and controllable manner if and when the need should arise. If and when then need should arise, slamming the nose down might be less than ideal.
they had less than 40 seconds to override MCAS.
Which means cutting the stab trim and then cranking the wheel, which Djacobow suggests it is maybe 1/3 to 1/2 as fast without power. If they had already allowed MCAS to turn the wheels all the way down, taking 20 seconds with power... That would be a fun challenge of how much motor coordination do you have while spearheading into the ocean. On one of those flights, I read the plane was pitched 49 degrees down. Even 30 degrees is pretty steep. San Franciso's steepest streets are around 30 degrees, if you've ever driven there. It's unnerving on a road.