Seems like a fairly accurate description of what happened based on known information. The over aggressive MCAS system with a AoA failure leads to the manual stabilizer becoming very difficult if not impossible to operate under specific flight conditions.
The manual trim wheels each have a fold-out handle. The handles are mounted 90 degrees of rotation apart to ensure that one of them is always in a position which gives the pilot good leverage to turn the wheel. This system is not unique to the MAX or even the larger 737 fleet. This was the backup trim system for all of the B707, 727, and 737 aircraft which all have long records of operating safely.
When the airplane is close to in-trim, as you would be throughout a normal flight, the wheel is easily moved by the pilot-flying (PF) using the fold-out handle. As you get farther out of trim, the forces required to turn the wheel increase so it might be necessary for the PF to call for trim and have the pilot-monitoring (PM) turn the wheel. i.e. "Trim down" ... "Stop trim"
When an out-of-trim airplane's airspeed increases the aerodynamic force on the stabilizer increase which increase the pressure on the stabilizer jackscrew which increase the force necessary to turn the trim wheel. If the airplane is significantly out of trim and at high airspeeds, it may be necessary for both pilots to work together to turn the trim wheels. The Captain using his right hand, and the F/O using his left hand, on the extended handles.
If you allow the situation to progress to the point that the trim is at the nose-down limit, and your airspeed is excessive, you may have to unload the stabilizer to be able to move the trim wheels. The stabilizer is introducing a strong nose-down moment so you unload by relaxing back-pressure and allowing the nose to drop. While the back pressure is relaxed, the forces on the stabilizer are reduced and you both trim together to progressively move the trim away from the full nose-down position. You alternate between unloaded to trim and raising the nose to regain the altitude that you're giving up in each cycle. With each cycle, the stab trim is moved closer to neutral and the forces required for the next cycle are reduced.
The key here is to avoid situations where you have full nose-down trim and excessive airspeeds. If you do, you'll never be in a situation where the more extreme measures are necessary to get or keep the airplane in trim.
The DFDR data from all three flights (Lion Air incident flight and the Lion Air and Ethiopian accident flights) show that the primary electric trim overrides MCAS (as it is designed to do) and can be used to keep the airplane in-trim through multiple unschedule MCAS activations. The Captain of the Lion Air accident flight kept the airplane in-trim through 21 unscheduled MCAS activations. Each time MCAS started trimming down, he applied nose-up trim with the thumb switches on his yoke which stopped the MCAS activation and allowed him to return the airplane to an in-trim state. There was nothing that would have prevented him from continuing to do this indefinitely. Unfortunately, he didn't continue flying the airplane. The F/O couldn't find the applicable checklist so the Captain transferred the airplane to him and began looking for the checklist in the quick reference handbook (QRH) himself.
While the F/O was flying, he had 5 additional unscheduled MCAS activation. He stopped the first 4 with a quick bump of the trim thumb switches on his yoke but he did not continue to hold the switches long enough to remove the nose-down trim that MCAS has applied as had the Captain. With each MCAS activation the stab trim moved progressively closer to the nose-down stop. After the F/O's fifth MCAS activation the stab trim was at the full nose-down stop and they were unable to maintain control. The stabilizer runaway procedure was never accomplished and the use of manual trim was never attempted.
On the Ethiopian flight the Captain was again flying the airplane. This time there wasn't a fault in the left AoA sensor, the sensor was damaged by an apparent bird strike almost immediately after takeoff. The effect was similar. The left-side stick-shaker activated and "AOA DISAGREE" messages appeared on both primary airspeed indicators. The Captain engaged the A autopilot (A/P) while still below the minimum A/P altitude engagement altitude of 800'. The A A/P draws its flight data from the left sources which were the ones which were compromised by the loss of the AoA vane. (If he had selected the B A/P it would have flown from the right data sources and MCAS would not have activated as that would have switch from the compromised left FCC and ADIRU to the right FCC and ADIRU which were unaffected) The A A/P attempted to follow the invalid left-side air data which proceduced the unstable initial climb which, prior to the release of the DFDR data, suggested that the problem had been something other than MCAS which is inhibited with flaps extended. After approximately 30 seconds, the A A/P disconnected due to the increasingly erroneous data it was receiving.
Once the flaps were retracted, the unscheduled MCAS activations began. The Captain re-extended the flaps and that temporarily inhibited MCAS. For some reason, the flaps were again retracted and the unscheduled MCAS activations resumed. Many of the MCAS activations were stopped with the primary electric trim but the Captain was not fully re-trimming out the MCAS trim inputs. Meanwhile, the Captain had called for the Level Change (LVL CHG) vertical mode on the Mode Control Panel (MCP). The MCP selected altitude was well above their current altitude so this put the auto-throttle system (A/T) into "N1" mode with climb thrust (CLB) commanded. The flight directors would then be commanding pitch attitudes to track the airspeed selected in the MCP window. In LVL CHG, the A/T will not be reduced to correct for overspeeds. The F/D, and A/P if engaged, will command higher pitch attitudes in an attempt to correct the overspeed. The A/T were left engaged throughout the flight and they maintained the very high CLB power setting. This high power setting caused them to accelerate quickly. Actual airspeeds (indicated correctly on both the F/Os and standby indicators) reached the 390 KIAS range. Vmo for the airplane is 340 KIAS and a more appropriate speed for their situation would have been in the 210 KIAS to 250 KIAS range.
Within a short time, the uncorrected unscheduled MCAS activations had progressively moved the trim to an excessive nose-down position. At some point the F/O recognized an MCAS/stabilizer runaway and the memory items were partially accomplished (they failed to disengage the A/T and never reduced thrust) including flipping the primary and standby trim cutout switches. This removed all electric trim from the stabilizer system but, when they did this, the trim was already close to the full nose-down stop. The Captain (PF) told the F/O to use manual trim but it is unclear if he tired to use the trim switches (now deactivated) or turn the wheel. If he tried to turn the wheel it isn't clear if he unfolded and used the handle. There was no indication on the CVR that the crew ever attempted to use both trim wheels together or to unload the stabilizer to reduce trimming forces.
At this point, they decided to try the electric trim again. They deactivated the two trim cutout switches but either failed to hold the primary trim switches in the nose-up position or the combination of excessively nose-down trim and excessive airspeed produced loads that were too high for the electric system to overcome. What did happen was that the unscheduled MCAS activations resumed and it pushed the trim to the full nose-down stop. At that trim setting, and around 390 KIAS, they were unable to maintain control.
The sim trials that have been reported have been in NG sims which don't have MCAS. You can't accurately simulate an MCAS runaway in an NG sim because as you introduce the nose-down runaway the pilot will instinctively apply back pressure which will cut out the trim with the trim brake. The trim brake is not active on MCAS activations because it would prevent MCAS from doing its intended job of introducing a nose-down bias in high AoA situations. Since they couldn't replicate the MCAS runaway, they started the trial with the trim already full nose-down and at around 250 KIAS. Those who have tried this have said that the recovery is difficult due to the factors that I've already discussed.
The key is not letting the trim get all the way to the full nose-down position before you take action to recover. Use the yoke trim switches to keep the airplane in-trim until the electric trim system is disabled.
