I think that elastic effect will not slow down if there are low unrelated skills. It just train them weakly too (if there is elastic ON), I suppose.
You mixed first and second interpretation together.
I think purpose of this change is to make cross-training little bit more effective (and easier as it would take little bit less time than before the change). But e.g. pops of REB during outside training it would make cross training even more difficult (due to salaries increase if you train outside skills for bigmen), so I hope interpretation of no additional pops and just boost of speed is right one.
And about thought that it would cause training on high level slower. It could be like this or couldn't depends on implementation if influence of unrelated skills is always positive or minimum boost should be 0. But I expect more likely negative effect on higher levels because without negative levels would be increase total global value of the training, but I'm not sure if that's what BB-staff follows.