CRM Online automated posting to Yammer not working as expected

I have configured CRM Online (2015)'s Activity Rules so that "New Opportunity for a Contact" is Active and "Post to Yammer Activity Stream" is set to Yes.  We have Yammer configured so that posts are sent to a specific group (Business Development), but also that the level of security for Yammer activity stream messages is Public.  We have two problems:

1: While Yammer messages we type into CRM Online, e.g. into the Contact form, show up in Yammer, they default to the All Company group instead of the Business Development group.  We have to manually remove the All Company group and add the Business Development group every time we create a post.

2: We expected that when a user creates a new opportunity for a contact, this would automatically create a posting to the Business Development group in Yammer.  Microsoft Dynamics support team is telling us that when we activate the "New Opportunity for a Contact" rule and set "Post to Yammer Activity Stream" to Yes, this doesn't mean that when someone creates an opportunity for a contact, it will create a post in Yammer. What they tell us is that it will only create a System Post, but it will also show up Yammer in the Recent Activity section.  I am puzzled on two fronts.  Firstly, "Post to Yammer Activity Stream" sounds like it should create a post in Yammer, in our case to the Business Development group.  I cannot imagine anyone thinking that "Post to Yammer Activity Stream" should only cause the New Opportunity event to only show up in System Posts.  Secondly, the new opportunity doesn't show up in Recent Activity either, even after a couple of hours.

Any help you can give me in understanding this will be gratefully received.