Ironclad reason: legilemency and/or veritaserum

[Eddie]

No: HPFGUIDX 171356

OK, I searched for "ironclad legilemency", "iron clad legilemency", 
"iron legil", "legil verita snape", etc, and have not found this 
theory.  Apologies if this has been done to death before:

Was Dumbledore's ironclad reason that he trusted Snape because he 
used Legilemency and/or veritaserum on Snape (with Snape's voluntary 
approval)?  Yes, Snape is a masterful Occulumens, but could he 
possibly block Dumbledore (!) from knowing the truth under such 
conditions?

Eddie