Regarding thread engagement: more won't necessarily help. In aluminum, you should have an engagement of twice the fastener diameter, so 20mm in our case. Unfortunately, the top two threads carry most of the load. If you now go to 30 or 40mm and increase the torque on the bolt, the top two threads will start to strip, no longer carrying and load. The load will now be on threads 3 and 4 which will also soon strip and so on. Obviously we are not at this point, otherwise the TTY bolt would also strip out the aluminum, but your finding of 20mm engagement fits perfectly engineering guidelinies.