I actually tend to suspect the opposite. Throughout the books, it's been a sign of character development when he shares things with his allies and talks about his emotions. It wouldn't make much sense if his teenage self was more mature in this department than the Harry we see in the books.
Also, remember that he'd just been betrayed by a father-figure (and the woman he loved), and this has been implied to be the root of his trust issues. I would find it very odd if he were suddenly completely open and honest with his new father-figure.
Oh, I agree that Harry wouldn't have "just" opened up. "New adult male authority figure? Cool, I'll spill
ALL my heartache and self-doubt!" ... yeahNO.
But in time, as Eb proved trustworthy...
I don't think Harry ever saw Justin as much of a "father" figure, however: he had memories of Malcom, after all! Justin taught Harry to Shield by flinging baseballs at him. We presume many of Justin's "lessons" were similarly abusive... but that Eb's were not.
Eb earned from Harry a singular level of trust & respect.
Why? What would that accomplish? He knows Harry is redeemable because he soulgazed him. What else would he need forbidden magic to find out that mattered so much?
DuMorne had been a Warden; he was part of the team attacking Kemmler, part of the magical bomb-squad going into Kemmler's lab afterwards. This implies a pretty senior Warden, and a lot of WC trust.
But somehow, 20-30 years later, he was so dark a warlock as to summon HWWB to kill an escaping apprentice?
Eb himself had been a Warden; he'd have been intimately aware of what a huge issue this was.
The
WTF must have been intense in the WC. The whole shitshow would merit as much information-gathering as possible... Is Justin really dead? Did Justin really summon one of the most-dangerous Outsiders known? Did Harry really defeat said Walker?
WTF was going on at Justin's house??? What else -- besides the corruption of a senior Warden, the attempt(s) to take a promising apprentice down his corrupt path -- had gone seriously wrong, there?
I really cannot see Eb being satisfied just with knowing Harry was "redeemable," this issue is MUCH bigger than Harry in and of himself. Outsiders are an existential threat; and the WC's Blackstaff exists specifically to break the Laws, when and if such threats need to be met outside the Laws.
Edit: Also, I cannot honestly see how the WC wouldn't know of DuMorne's other apprentice. They investigated the events, and I cannot believe Harry would have been able to recount a credible lie (nor why he'd even have tried) that omitted Elaine's role in trying to help DuMorne enthrall Harry; he'd have still been shaken, and in accute mental/emotional distress; they would have been cross-examining him, discussing his likely death right there in front of him. This does not create the environment for Harry to tell a lie... especially one he'd see no need to tell! It's not like he thought he was protecting her!!! The WC dropped the "Elaine issue" simply because Harry was absolutely convinced that she died in the fire; his certainty of that was what convinced them.