I sort the results by posts instead of topics more than 90% of the time. I don't see how it will hurt since that view shows the topic title and a link to it. One gripe is that it only shows a snippet, something that I never liked about vbulletin. Please change it to show the entire post with scrollbars, like it used to. Also a good idea to allow the user to save search options with a checkbox on the search form page.
Yeah, this will be a good feature. With email, you won't know if they've read it not. I always prefer to use PM for more control (ability to unsend, track).
I think it's better to have it like before, one name per line. Also, change the message to use commas to separate: You may add up to 5 other recipients, separated with commas.
And why remove the ability to hide other names in the CC field?