If he could do that, he wouldn't need to be able to do that, because he would have already done it to others. We'd be lucky if there wasn't a collective Kemmler hive mind running around fucking everything up.
Excising a relatively small portion of a spirit's memories is a far cry from drastically reprogramming what is likely the most intricate and powerful working of mortal magic in the current iteration of Creation, at least on Earth.
"Ability to understand humans" is not a relatively small portion of Alfred's memories. It is likely one of the cornerstones of island's security. The Warden is a human. WIthout ability to understand humans, Alfred isn't able to understand Warden's motivation fully, and likely cannot enact whatever contingencies there may be in case of a rogue warden acting counter to the prison's purpose - which likely was Kemmler's goal in the first place. The selection of the next Warden would also be compromised without ability to understand humans and thus check their character.
As to doing it to others - this is only an example. Creating various mental commands, backdoors, compulsions, blindspots etc are all in the realm of possibilities. As to the concern about ego subversion - it might require Demonreach infrastructure.
And it's not just Kemmler I am woried about. Note how I worded the question. "Warden of Demonreach is free lord under the Accords" is a possible consequence of becoming a Warden Alfred wouldn't know about. "Wardens of Demonreach owe X fae an inherited debt" is also a possible consequence of becoming a Warden. This is a legacy position that comes with benefits, obligations, politics, and dangers that we should learn about. Thus, the question. It doesn't cost us anything, as it uses the scene.
This didn't happen in canon and Dresden was Warden of Demonreach then too. This doesn't even sound possible. It would require that Kemmler be able to subvert a creation of Merlin so thoroughly he'd have to be beyond him entirely.
I don't want to rely on meta knowledge here, plus see above - I might have used a bad example, but there are tons of things that Alfred wouldn't know about or be able to tell Harry that come with the position. Like, for example, it's possible Harry just inherited some bank accounts or real estate, or debts.
EDIT:
To summarize, when I use "consequences/effects" I mean:
1) Magical consequences. How Harry's magic is going to be affected. Harry is a Starborn, and he might be the first Starborn Warden of Demonreach. Some interactions might be happening that Alfred is unaware of or at least not expecting to happen. We also know that Harry cannot claim anything else until he dies. There might be other consequences too, like different affinities becoming more or less pronounced, certain types of magic becoming available to him (maybe Ancient Sorcery?) etc.
2) Mental consequences - broadly those would be any and all possible compulsions, backdoors, blind spots, geases etc that Harry might have been exposed to. These might have been created by Kemmler, who subverted Alfred in at least some respects, by Merlin, who we know broke Laws when making the prison, so installing mental effects into the Wardenhood aren't out of the question, some of the prisoners, if the security is not totally perfect, or some other agents.
3) Political consequences - the Warden might be afforded place among senior council by default, they might be a part of certain treaties, etc. The position might owe certain debts to, or be owed certain debts by various magical beings and factions.
4) Matters of inheritance - i wouldn't put it past some of the previous Wardens, including Kemmler to have things (money, artifacts, real estate, etc) willed to their successors.
We really want to check for all of this, and the question is a good move for that.