Now learn the dative case. Suppose I say to you "Give me your book." The word 'me' is in the dative case because what I really mean is "Give TO me your book." The dative case is when you have in, to, by, for, from, and so on. Those are prepositions. 'Whom' is the dative of 'who'.
Now learn the pronoun. I, you, he, she, it, and so on are pronouns. Each refers to someone or something already named (or implied). A pronoun refers to the last preceding qualified noun.
Now we can sort this out.
"Her mother, in whom she confided, said (that) she would support her unconditionally."
The first "her" is a possessive introducing the subject, "mother". It refers to an unnamed person.
"She" refers to the same person as "her". To reverse the meaning it would have to be "Her mother, who confided in her, ..." (And then "her" would be the dative case.)
"That" introduces a clause, optional in English.
"She" refers to the subject, "mother", because that is the only qualified noun.
"Her" refers to the same person as the first "her", who so far has not been named.
BTW you have two questions here. You really should post them separately to avoid confusion and spread the joy among responders.
"Can ya please tell me which sentence is right:
The man who thinks (that) he knows how to play soccer,
OR the man who thinks knows how to play soccer well."
These are two different sentences saying different things. It only depends on what you are trying to say.
