Yes it does display Mach Tuck but it is much less than the 200 series, and very gentle nose down. It was fully controllable. Sorry dont know about with the autopilot on.
I would suspect that the AP would maintain the altitude until the AP force limits are exceded and it disengaged. Probably not a good situation.
Have not done a high speed RTO but done a few max effort landing in both the 200 and 300 in dry conditions. The 200 did judder at the lower speeds but at the higher speeds were fine. The 300 was smooth as a babies right to the stop, however lateral control felt like landing on a very wet runway. The first time was a shock at how well the brakes work. The jet had come to a stop before reverse had got to accel past the detent, the cause was partly the 'startle' factor but the rest was how awesome the brakes worked.