Questions on the Heads of Houses

[terri-testing.livejournal.com]

So, a couple of questions I wanted to toss out about the Heads of Houses.

First, Snape, the Head of Slytherin, seems to be much younger (31 when we and Harry meet him) than the other Heads of House.  But, at (by?) that time he seems to have the support of his students (as well as supporting/defending them).  In particular, in second year we see Draco Malfoy sucking up to him by suggesting his father (who is on the Board of Governors) would support Snape's possible candidacy to be Headmaster.  

On the other hand, someone (Jodel?  Whitehound?  Swythyv? Oneandthetruth?  I'm not remembering at this time) suggested that Snape might have been installed as Head of Slytherin to weaken the house (given his dependence on Dumbledore).  (And that the loss of the Slug Club funnelling fresh talent into the Ministry did fatally weaken it to the point Voldemort found it possible to topple it with one blow.)

So.

Do we have canon how much Snape's house supported him?

Do we have canon when he was actually installed as Head?  I always assumed he replaced Slughorn as Head of Slytherin and Potions Master simultaneously in 1981, but that's an assumption.  So my assumption Snape was to credit for that long string of Slytherin House Cup victories might not be correct.

And... the Board of Governors has something to say about the headmastership.  What about House Heads?  Was Snape, whatever year he was installed as Head of Slytherin House, installed with their approval, against their will, or what?