I don't think it has anything to do with Mab's strength... I am willing to be that Maeve was allowed to pick her Knight just like Lily was able to chose Fix.
I agree with you there. In normal circumstances, it is the lady that select the knight and in most circumstances the queen did not interfere to much.
Now, the questions: Why does the queen not interferes in a knight selection? Is the selection of a knight is beneath a Queen's notice?
We know that Slate is weak. And I assume we agree that Mab knows about Slate's weak nature before SK. Why then, did she not order Maeve to replace Slate? If a knight have to be powerful and capable like Harry and if such a knight is critical for the winter court as a whole, how come a mediocre knight like slate can hold the job?
I understand why Titania did not interferes with Lily. She simply didn't care. But Cd show that Mab did care about Maeve. Why then, did she let Maeve have a weak knight?
The only answer that I can think of for these questions is the conclusion that a powerful knight like Harry is undesirable under normal circumstances. It is to much trouble to kept a knight like Harry unless there is a definitive problem that needs such a knight. This let me to think that Harry may be in danger once he fulfills his duties to winter. He won't be needed anymore and he will starts to be more trouble than he is worth, But since Mab is a strong Queen herself, she'll probably find a way to make use of a dangerous knight like Harry. A less secure and capable Queen however, might view Harry as a threat and might find it easier to terminate him either by killing or use drugs and seks to keep him insensible.
Butter's comment about the mantle shutting down Harry's ability to feel pain is Mab's way to control the knight supports this theory. If Mab have to regulates the knight so he won't grow to powerful , it means Mab have reason to fear her knight.