The other question I'd love an answer to is why are the bolts "always
replace." Does the torquing process (20 Nm + 90º) weaken the bolt, or
is it not reproducible? Or something else altogether?
For a conventional bolt, ignoring the torque required to overcome friction when you tighten it, there is a linear relation between the amount of torque and the clamping load in the bolt. If you double the torque, you get double the clamping load. But the actual clamping load is not very well defined, because the amount of friction is unknown - it depends on the surface condition of the parts, any lubrication applied to the parts, etc.
T2Y bolts work in a different way to give an accurately controlled clamping load. When you tighten the bolt, it is deliberately overstressed so that the bolt permanently stretches. For this to work properly, it depends on two things: you need to apply enough torque to permanently stretch the bolt, but not so much that you over-stretch it, which would weaken it or even break it.
The specified torque load (20 Nm for your bolt) is enough to get "close" to permanently stretching it, even if there is some friction in the joint. Actually the torque level is not too critical - the main reason for specifying a value is to make sure that you have taken up all the slack in the joint before the next step.
The second part of the operation, turning the bolt through a specified angle (90 degrees) is the critical part that ensures it really is stretched - but not by too much, because the maximum stretch can't be more than 1/4 of the thread pitch, if you turn the bolt by a 1/4 of a turn.
If it is installed properly, the clamping load will depend only on the diameter of the bolt and the material it is made from, and not on uncontrolled factors like friction.
The stretch is permanent - if you measure the length of the bolt, install it and remove it, and measure it again, it will be longer than before.
If you re-use the bolt following the same torqueing procedure, you will stretch it by another quarter of a thread pitch - i.e. the total stretch will be twice as much as it was designed for. That may not have much effect on the clamping load (indeed it might increase it rather than decrease it), but the stretching changes the structure of the material in the bolt, and it will be more likely to crack and/or break.
In fact, stretching the bolt with a 90 degree turn, releasing the stress, and then stretching it again would be likely to cause more damage than one continuous 180 degree turn.
So the take-home message is: these bolts are for single-use only!