I believe you are asking a detailed question where most of the time we only answer if it is easy to do.
Answer to the first question
Using group theorist's notation so I don't get lost:
$$[\theta,\phi] = \dfrac{1}{|G|} \sum_{g \in G} \theta(g) \overline{\phi(g)}$$
We have the nice relation: if there are non-negative integers $d_\chi$ so that
$$\theta = \sum_{\chi \in \operatorname{Irr}(G)} d_{\chi} \cdot \chi$$
Then we have the nice projection / use dot products to get coefficients:
$$[\theta,\chi] = d_\chi$$
and the standard sort of pythagorean thing
$$[\theta,\theta]
= \left[ \theta, \sum_{\chi \in \operatorname{Irr}(G)} d_{\chi} \cdot \chi \right]
= \sum_{\chi \in \operatorname{Irr}(G)} d_{\chi} \cdot [ \theta, \chi ]
= \sum_{\chi \in \operatorname{Irr}(G)} d_{\chi}^2$$
So, I think this means 1. No, there is no typo in the formula (or at least there is a similar formula with no typo, and it does have squares)
Answer to the second question
So how do you determine the $d_{\chi}$ from the sum of their squares? Well, this is pretty hard in general. It is like finding the angle inside a circle from its radius ... you can't in general. However, we have assumed the $d_{\chi}$ are non-negative integers, and as you have mentioned, often there are not many possibilities.
- $1 = 1$
- $2 = 1^2 + 1^2$
- $3 = 1^2 + 1^2 + 1^2$
- $4 = 1^2 + 1^2 + 1^2 + 1^2$ or $4 = 2^2$
- $5 = 1^2$ plus one of the fours
Unless the number is small, you don't have much hope. However, the number is often small, so this trick gets used a lot.
A better version not only computes $[\theta,\theta]$, but also $[\theta,\phi]$ and looks for all the possible $d_\chi$ for $\theta$ and $\phi$ that would create that matrix. GAP has a command for this, OrthogonalEmbeddings
and it is super-fun when it works (as in, gives you only a single solution -- usually it gives you many solutions).
Sorry on the third
I'm not sure about your third question. You might find those numbers early or late in the process. I think of them as "decomposition numbers", but that might not be the right word in this case. Another word is "multiplicities of the restriction".
Sometimes that is a good idea. In a recent question I asked, I need 7 positive integers that add up to 7 and that include 7 copies of "1". So I guess that was easy? But the next part needs some positive integers that add up to 14. I don't know how many (maybe I know it is 1, 7, or 14?).
I think the case you are studying is called spin and often the questions you ask are very successfully answered. A more general version is clifford theory and is also very successful. The even more general version is just induced characters. I am not having much luck personally there, but I am hoping my question will be Clifford theory once I figure out the next step.
Vocabulary
compound character is "reducible character"
group embedding is fine, or you can say "subgroup" if the way the two groups are related is clear. "fusion" is the name of how the conjugacy classes between the two groups match up (the way you added Class 2 and Class 3 of A4 to get Class 2 of S4, and the way you got 0 for Class 2 and Class 5 of S4).
Best Answer
$H$ contains the element $(13)(24)$ (the square of the generator $(1234)$), which is of type $C_2^2$.