How do you prove trustworthiness?

[Richard]

No: HPFGUIDX 145979

Frankly, I'm not on any side, when it comes to Snape being DDM!, 
ESE!, OFH! or anything else you care to mention.  I think it is up 
to JKR to tell us in her own good time where Snape ends up in the 
tale she is telling.  That doesn't mean I'm uninterested.  There 
have been some good arguments in each direction, with one hole that 
I've not seen filled with regard to the DDM!Snape theorists.  (The 
prima facie evidence for ESE! is, I believe, sufficient in its own 
right.)  Perhaps I've just missed the posts where this hole has been 
filled ... any easy thing, considering the fact that I don't have 
the time, nor the finances, to make reading posts a career.  My 
apologies to others if this is something I've missed ...

So, let's assume that Snape is either DDM! or OFH!(but anti-LV) 
(let's call that one OFHbaLV!Snape.  It thus behoves Snape to 
eventually find a way to prove his reliability (at least with regard 
to trying to kill or otherwise defeat Voldemort) to Harry.  So, how 
does he do that?  If there is no plausible way for him to do this, I 
think it puts a real, and perhaps fatal, hole in both the DDM! and 
OFHbaLV! theories.

The best way that has been mentioned (or at least that I've seen 
mentioned) is a combination of Portrait!Dumbledore and Dumbledore's 
memories a la Pensieve.  I think this weak because of the fact that 
Harry saw Snape "kill" Dumbledore, and is now convinced that 
Dumbledore was tragically mistaken with regard to Snape.  I don't 
think even Portrait!Dumbledore telling Harry, "You know I was dying, 
so I told Snape to finish me off so he could see that all the Death 
Eaters left Hogwarts without harming any students ... including both 
you and Draco," would really accomplish this.

What would convince Harry that Snape should be trusted to any 
appreciable extent?  I think Portrait!Dumbledore would have to play 
a part, and a couple of Pensieve memories from Dumbledore would 
help, but would Dumbledore have been sufficiently prescient to have 
included "Why I trust Snape" memories set aside for Harry?  Or was 
what Snape had shared with Dumbledore of such a nature that 
Dumbledore would not feel it proper to share it with Harry?

About the only way I can see through this problem is a combination 
of Portrait!Dumbledore giving assurances to Harry and getting Harry 
to use the Pensieve filled with Snape's memories.  But, which 
memories, and how would Snape (or anyone else) be able to control 
the sequence that Harry viewed them (which might make a major 
difference in their interpretation).

There is one way for Snape himself to control the sequencing of 
memories that would convince Harry that Snape really is working 
against Voldemort: He could give Harry a bottle containing his 
(Snape's) memory of re-experiencing those memories in a Pensieve.

Imagine a scene where Harry enters a Pensieve containing this 
memory.  He falls into the memory, and finds himself seated across a 
Pensieve from Snape, who (with all the usual snide and bitter 
comments) explains why he did what he did, then sequentially pours 
his own memories into the Pensieve ... and dives in.  As the 
experience of reliving these memories would be part of the memory 
Snape bottled, we have an doubly indirect experience of these events 
by Harry in precisely the order Snape wishes them to be presented, 
and with suitable commentary and explanation from Snape as fellow 
spectator.

Would this be sufficient to re-establish some level of trust in 
Snape (for purposes of combatting Voldemort) in Harry that DDM! and 
OFHbaLV!Snape's are plausible?  I think so, but would appreciate 
comments from others about other ways of keeping all the DDM!, OFH!, 
ESE! and other Snape's in play.  (I think, for the purposes of JKR's 
tale, it is actually important for her to keep all these options 
open, so as to maintain dramatic tension, which is why I'm willing 
to let her decide how and when to resolve this.)

Richard, who enjoys a good theory, provided it makes sense.