I've recently played with Yog-Shub. One of the tricks I run is:

Living Mummy

Forced Response: After Living Mummy enters your discard pile from play, wound all characters.

with:

Shocking Transformation

Action: Sacrifice one of your characters to search your deck for a non-Ancient One character and put it into play. Then, shuffle your deck.

and
• The Necronomicon: Olaus Wormius Translation

To wipe the board and get some characters of my own at the same time. I think it works as cost is payed before effect takes place. But then i thought about response window that happens really late. And then forced response happens faster, but maybe still not fast enough to go between paying the cost and the effect itself. I'm not 100% sure it works anymore. What do you think? (I hate CoC timings..., love the game though)

Also when i hit:

Expendable Muscle

If Expendable Muscle would be wounded or go insane, instead attach it to a character you control. Then, Expendable Muscle becomes an Attachment support card with the text: "Attached character gains Toughness +1 and Willpower."

Can it protect another character when mummy hits them both? Passive effects have really weird timing...

Regarding Living Mummy, I think what happens is that the effect of Shocking Transformation (or Necronomicon) is resolved, the character (or characters) are put into play, then the Forced Response is triggered.

Using Expendable Muscle to protect a different character should work, since:

Whenever character or support cards enter or leave play at the same time, the controlling player chooses the order in
which they enter or leave play.

The Living Mummy + Necronomicon scenario is covered in the current FAQ, and confirms jhaelen's answer.

How does Living Mummy interact with the Necronomicon (Olaus Wormius Translation)?

A Forced Response is resolved as soon as the entire effect it is responding to resolves. So Living Mummy’s effect will resolve after the Necronomicon’s ability resolves, completely.

I must admit that doesn't make much sense to me. I personally think the timing structure should also permit Forced Responses associated with the action initiation (i.e. with the paying of costs, like the sacrificing of Living Mummy and shuffling back of The Necronomicon), following Disrupts, instead of only referencing Forced Responses after action execution, but them's the rules!

On a related note, the timing structure also fails to include Passive abilities until the action is executed, however there is a note that states this should be otherwise elsewhere in the FAQ, of:
"NOTE: If a passive ability would alter an action as it is being initiated, the passive is first resolved on the action, which now altered, is initiated."
I reckon Forced Responses deserve a similar addendum for action initiation, but alas...

I feel a bit stupid for missing the FAQ describing this interaction directly.

Thx for clearing it.