That is a terrible rulling. Deny existence shouldn't negate Diana's weakness. The point of ignore vs cancel is that if a card has multiple bad effects you simply ignore one of them and do all the rest. It wasn't written that way to wiggle out of a signature weakness such as this. If you want to use bad templating as an excuse to give yourself an easier time that's fine, you can play the game however you want. But it's pretty obvious this isn't what they had in mind with Deny Existence and simply forgot to to add "or ignored" to the weakness.

From the above official answers:

Q: Luke plays Read the Signs to investigate a connecting location with a ready, unengaged Young Deep One. Does he take 1 horror? (I think no, because he is considered to be engaged with enemy, but there is no timing point of engagement.) A: Luke playing Read the Signs on a connecting location would not cause him to engage Young Deep One or take 1 horror.

Ouch. Although technically correct, this answer fails to address the fact that you would still incur 1 damage and 1 horror in the form of an opportunity attack from young deep one. Failing to include this information in an official answer just leads to confusion and could cause players to incorrectly assume they can play this card and other events without possible attacks of opportunity. They wouldn’t get the 1 horror since there is no engagement timing point to take it, but they are still considered engaged with young deep one and therefore will incur the opportunity attack.

After some testing, Dr. Charles West III is... inconsistent.

There are two main benefits to the doctor. The first is that he gives an additional hand slot, which can only be used for tools. This means you're trading an ally slot for a tool slot, and which encounters the usual problems of cards that take up slots for slots - you need extra actions to fill up those slots. Now, Seekers will effectively have a free hand slot, since most can justify running Magnifying Glass to fill it, and there are other useful tools that could be use - but that still needs you to spend more resources and actions to fill the slot, which is never great when your actions can always go towards investigating.

Second, on some successful investigates, he can exhaust to deal damage - but only on investigates that only succeed on 1s and 3s. This is two levels of specific - first, you need to succeed by 1 and 3. This is doable, but rather inconsistent - sure, you could use Steady-Handed to improve your odds, but it's hardly a guarantee, and Steady-Handed (and its other synergy, Chemistry Set) already encourages you to succeed by a certain amount. Second, this requires you to be at a location which has an enemy that can take one more damage.

There are very specific cases where he can be useful, but they require some fairly niche builds. If not for the differing classes, I'd be hard-pressed to find a case where you'd want to use him over Alice Luxley. His Science trait is the only notable one, and that's of limited use - he's not even Miskatonic, so he won't be getting any Miskatonic Archaeology Funding And with so many good Seeker allies available as is, you'd want a pretty specific place to fit the doctor in.

Here's a gist of Kate's usage of clues :

Get your 1st clue and put it on Flux Stabilizer to flip it active. This one won't get you the +2 yet, just the flip. Don't forget to put version of Aetheric Current you like into the deck.

Play some Science or Tool assets in different slots. (e.g. Flashlight/Magnifying Glass on Hand, Dr. Charles West III on Ally, Chemistry Set on Accessory, or even First Aid on no slot?) Each one let you get +2 on demand when putting a clue on them. It means she can get 4 or 4 from time to time too, or even more if you place 2+ clues on different assets at once.

Try to make the asset clue-less again, so you can do more +2 :

• Spend clues as a part of scenario progress, and choose the clues on assets rather than on the investigator card. So turns out, if scenario allows multiple investigators to contribute the clues to advance, Kate want to spend exactly clues on her assets and not clues on her investigator card, so she is ready to do +2 for whatever coming up. You can do that more if other players can flex to get some clues.
• Find the jackpot Aetheric Current in the deck and an unlucky enemy to unleash the current on. This makes all assets clue-less, provided that you have 1 clue to flip back Flux Stabilizer.
• Get to make one asset available.
• Failing Failed Experiment and choose the clue drop option. You can drop clues placed on your assets, and use some left over clues on investigator card for the +2. (Not necessarily for picking the clues back up.)
• Other creative plays, like picking Magnifying Glass (1) up and immediately play it back to make use of your leftover clues. (If dropping the clue that was on it is not a problem.)

If you put a clue on limited use assets like Flashlight, beware if you spent all the supplies then you want to switch to other Hand asset wanting to replacing it, the clue on the empty Flashlight would drop. Same for clue on Chemistry Set then it get discarded due to its own effect.

On testing Failed Experiment, placing a clue to get +2 would be effectively just +1 because test difficulty also increases. For this reason it might help to pack other boosting solution into the deck like Inquiring Mind, Esoteric Method, etc. Or simply heal the horror back with Steady-Handed, Logical Reasoning, etc.

If you succeeded the test by exactly 2 before triggering , you cannot get healed since it requires you to adjust the value up or down by 1 before checking the heal criteria. If you are using the card for horror healing purpose, you need to succeed by either exactly 1 or exactly 3.