I still think the chance is greater that he retained all of the Word. It isn't like Harry didn't already have existing talent, powerful talent, and as a star born to boot, I think he does retain all of the Word.. And knows how to use it. Consider this when Lash translated the Word for Harry, she wasn't dealing with a blank slate like she was when she translated Ghoul for him. Harry would have already understood all of the concepts, most likely knew them already save for the forbidden bits.
It depends how complex and specific the necromantic/ectomantic/etc arts are, and how much they overlap other branches of magic. Certainly EvilBob took Harry completely by surprise; the spell sort of "crept up" on Harry, and he didn't recognize it; Harry
does recognize most magic, had (for example) been able to detect Entropy Curses as they approached, even (to an extent) manipulate them.
I think the term "the True Art" was used, implying a scope to Necromancy that Harry's training won't have covered.
I go back to the text, where Harry and the Shadow had about "an hour" (subjectively) to discuss the entirety of
TWoK.
I don't think that's enough: just discussing the Darkhallow, ways and means to disrupt it, the risks and threats, etc... the scope of that threat is save-thousands-of-lives / stop-a-dark-god level of discussion, and you just don't cut corners with that sort of thing! So, a full tactical/strategic briefing. I'm pretty sure that's most of the hour, right there.
Because it was in German, needing Lash's translation, there's not really any chance Harry "retained" anything she didn't discuss with him.