Well from my understanding, Toyota would have a fail safe system on not overdoing the tranmission and engine.
If your on a higher gear at low speed, the car would shut off.
Just like it won't let you shift down from a higher speed.
So that could be the reason why you can't shift up
I was in 4th at 20 mph, and down shift a few gears. What was said possibly rpm and speed has to do with it, meaning if I was faster to go and shift so one it switch over, and that is my next test.
I looked at the video. It looks like a minimum speed to prevent lugging but if you are light on the throttle, there shouldn't be a reason for the PCM to think that.
Nothing weird. The electronics are just protecting the engine from lugging. It's a small 4 cylinder engine, not going to shift into an uncomfortable zone.
Yes, it's controlled electronically so that you don't shift too early or too late. I think it's kind of high also. If you want to shift earlier from 1st to 2nd, accelerate at a medium to fast speed up to say 15 mph (before the typical 18 mph) and let go of the gas then shift like if you where in a manual, then just accelerate as you shift between 2-7 (don't accelerate and let go of the gas like a manual). From my observation, the car senses less stress on the transmission and will let it go to 2nd gear slightly before (~2-3 mph) the 18 mph electronic "requirement".