something to eat is the direct object of your sentence (us being the indirect object, you can replace it with for us).
For that reason, your version a) does not work in this sentence.
If that were the meaning, the sentence should have been:
She has made us all eat something.
So, yes, you can put all after a pronoun used as an object. In this case, it is put after the indirect object of the sentence. The sentence would become ambiguous by putting all in between the indirect and the direct object if "all something to eat" would make any sense. But it doesn't, so we parse the sentence as:
She / has made / us all / something to eat.
Simply because parsing it like:
She / has made / us / all something to eat.
Makes our head spin, trying to combine "all" and "something" to "eat".