AppleScript error with if statement
Unless indicated otherwise, the comparisons are performed in order from left to right, so you just need to group them, e.g. x and (y or z) instead of x and y or z, otherwise that last or can decide:

if GUID is equal to FoundGUID and (trustitemOnMyList is equal to "TRUST" or trustitemOnMyList is equal to "2step")
    -- or --
if GUID = FoundGUID and trustitemOnMyList is in {"TRUST", "2step"}