Finally, let's check your example:

Yet I didn't decide, whether/if people with different character (can) have a common future.

It is not a conditional sentence.
Try rephrasing it as an "if-then" condition:

If people with different character (can) have a common future, then yet I didn't decide.

Does it make any sence? I believe not

"people with different character (can) have a common future" is not a necessary condition for "Yet I didn't decide" to be true. So, they cannot be joined using "если".

And below is an example where "если" and "ли" are contrasted, to illustrate how it can affect the meaning:

Я спрошу его, поедет ли он в Москву. - I will ask him if he goes to Moscow. = I will ask him: "Will you go to Moscow?". This sentence explains WHAT I will ask him.
Я спрошу его, если он поедет в Москву. - I will ask him if he goes to Moscow. = If he goes to Moscow, then I will ask him (something, I do not mention what). Otherwise, if he does not go to Moscow, I will not ask him. This sentence explains a condition WHEN I will ask him.