Thank you for posting these questions here! It'll be good to start gathering together some useful questions and answers for this game.
1) In some of our games the sequence of the optional characters is more important than it is in others, depending on how the plots are structured. In A Will to Murder, the sequence we give is preferable, because of the Lady Henrietta Crane case – if eg. Inspector Harris is added in before Atkins or Dr Barker, it's not possible for the Inspector to solve that case. But that isn't a complete disaster for the game – you would just need to be aware that that character wouldn't be able to achieve that particular goal. So I suppose the answer is that while the game will still work fine if you get a random selection of people present from the optional characters, you should preferably give Atkins tot he one who's most likely to attend: and so on.
2) Yes, that's right, that's what you should tell them. We have the rule like that specifically to reduce the amount of violence that might otherwise happen early in the game. It might still be worth attacking someone early on, because if you succeed and they are unconscious, you can take all their items from them. And, if you've used poison, they may not know who it was who was responsible. But in general we would expect people to save up their attacks for the final phase.
3) What you say is all true – naming their killer is a perfectly OK way for the player to use the WYLDB card, if that's what they want to do. Of course, as they're now dead, it can't really be considered legally binding evidence. And listeners won't know if it necessarily was the truth: they might be naming someone innocent, to get them into trouble. Or the surivors might agree to hush up this latest murder, if the murdered person was not liked. Or the victim might even feel that making some other dying remark, such as a pledge of undying love, is more important than accusing their killer. But basically you have to trust the players' own sense of what their characters would do in that situation. By the stage in the game that people are dying, they'll be enmeshed enough in the flow of it that they should be able to make the right decisions about what to say.
(Or, if you're really not sure about it, you could just not use the card: we don't always do so. The game works perfectly fine without it.)
I hope that all helps – do please come back with any more questions, these ones are very interesting and thoughtful!