Well, point by point:
Open new contact window: I see several problems. People might think it means add a new contact to the list, add/invite a contact to the conversation or open a new conversation with this contact. This, ofcourse would be partially solved with accurate tooltips and titles, but still it would not be as intuitive as it should. Also, it takes one double click on the Messenger tray icon to open the contact list and choose the contact. Send an instant Message is not modified by Plus!' Custom Contact Names, so what users with that option enables (quite a few), will only see email addresses, like in the invite box. Also it is much easier to open a conversation from the Contact List. It would be a problem, in my honest opinion.
Close button: Exactly below there is an exact equal button that closes the conversation/tab. If any use, it had to close _all_ tabs. However this would be very dangerous, because it may be mistaken with closing only a tab with catastrophic effects.
Detach button: I'd agree on that if it wasn't for the next reason: It is hardly used and there's already a mouse combination to do it (Middle-click, but probably double-click in the future beta versions)
Navigation Arrows: They already exist. Not in a form of "navigating through tab groups", but scrolling across tabs that ay be hidden outside the visible tab bar for space reasons.
Sorry to be skeptic, but we're testing it and I can tell you what is needed, what not, and what would annoy.
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