> And I would question: why would someone use an alternative pronoun, knowing that the person would prefer an alternative?
The answer is quite simple, we don't always keep track of other people's preferences even when we've been told them.
> Why would anyone choose to knowingly attack and hurt someone, purely for sake of labeling them with a pronoun that the person has identified as inappropriate?
Again, just because you had a piece of information, doesn't mean it immediately comes forth in your mind at the necessary time. It's hard enough remembering the preferences of your friends, let alone public figures who you've never met. No one here likely personally knows Manning, to expect people to remember the preferences of public figures they don't know is expecting too much.
> When someone has identified their preferred pronouns, it is entirely appropriate to use those pronouns, and it is inappropriate - even abusive - to deliberately choose to use an incorrect pronoun.
No, it is not. You are placing an unjustified exception on another persons memory of your preferences, this is never appropriate.
> I do state "deliberately choose to use", because there will inevitably be people who mistakenly use an incorrect pronoun, for lack of knowledge, but people who are aware of the correct pronouns and deliberately choose an incorrect pronoun are conducting a bullying, abusive attack, and should be stopped.
This is a false dichotomy that presumes perfect memory; just because you've been told something doesn't mean you're deliberately ignoring that information when you later use the incorrect pronoun. Some people don't know, but knowing doesn't mean you deliberately intended insult when you still use it wrong. We all know lots of things that don't come to us in the necessary moments, brains aren't computers, they're fuzzy.
If your ability to cope depends on another persons choice of vocabulary, you're depending on the wrong things.
Anyone who claims to be offended by "they" is someone playing victim, they're looking for reasons to be insulted. They/them is always appropriate in all cases for all genders, it's how we avoid having to remember everyone's personal preferences.