Relative pronouns are complicated so I can understand your frustration...
I will try and make them easier for you to use with some Rules of Thumb designed to help -
They will be both followed by a verb (sometimes by ne) , they will certainly never be followed by a subject pronoun or the name of a person ( je /tu/ il/elle/on , ils ,elles, Marie, Papa etc..)
C'est elle qui va aller en courses = It's her who is going to go shopping
Les enfants qui jouent dans le parc sont les miens = The children who are playing in the parc are mine
Je me demande ce qui va arriver = I wonder what is going to happen
You will however find object pronouns sometimes after qui/ce qui:
Je fais ce qui me plaît! = I do what I like
Elle a choisi cette tenue, ce qui lui va très bien = She's chosen this outfit, which suits her very well
C'est ce qui *vous * attire chez lui? = Is that what attracts you to him ?
*vous * note here is an object pronoun.
Also note, *qui/ce qui cannot be elided .
Que /Ce que
These relative pronouns will be followed by subject pronouns, names ...:
L'homme que vous voyez devant la Poste est mon cousin = The man (that) you see in front of the Post Office is my cousin
Le train qu'il a manqué est parti en avance = The train (that) he missed, left early
C'est ce que m'a dit Martin = That's what Martin told me
C'est Daniel qu'elle a vraiment aimé = It is Daniel that she really loved
Je vais vous dire ce qu'on va faire = I'll tell you what we are going to do
Note: Que/ce que can be elided to, qu'/ce qu'
C'est ce que son mari a décidé de faire= That's what her husband decided to do
This is why your first sentence is incorrect as it should be -
Tu ne devineras jamais ce que sa maman a fait !
Hope this helps!