Re: abusive sensei
I think people sometimes make the mistake of thinking you can describe 'abusive' training on a line of training intensity, that as training gets more intense (or physically demanding) it inevitably gets closer to abusive and it's just a matter of where you draw the line.
But IMO, it's not at all. Tough or physical or intense have NOTHING to do with abusive, nothing at all. It's not simply a matter of 'this intense is hard training, go a bit farther and it's abusive, depends on the individual how far they want to go'. That's one of the mistakes that leads people to accept abuse, because they convince themselves (or someone else convinces them) that it's what happens when you want 'really really hard training'. E.g., that injuring a trapped uke or terrorizing people junior to you somehow makes it 'hard training'.
You can train extremely physically hard, even 'aggressively', and have absolutely nothing abusive about it, and train much less physically 'intensely' and have it be abusive.
It's the relationships between the people, the psychology used, the way power differentials are used, the way consent is ignored or coerced... this can be done without much (or sometimes any) physical violence at all, similarly physically very difficult training can be based around psychologically healthy relationships and attitudes.