Q1 I don't believe that Microsoft has updated public guidance on this yet. Given that it's mid/late June and New Yammer is still in Preview I'd say that the July date is not a sure bet.
Q2 Only community admins can pin/unpin
Q3 No expiry
Q4 Community Admins only
Q5 Yes. The Cover Photo is visible in all end-points that use New Yammer (mobile, New Yammer on the web, MS Teams Communities app).