More is a indefinite pronoun in allone of your examples, but much and a lot are not adverbs here.
In:
I can't stand much more of this.
..."Much" is not an adverb. It is a determinative, and it in turn modifies the indefinite pronoundeterminative more. This means that much more is a noun phrase and it quantifies the pronoun this which is the object of the preposition in the comparative phrase more of.
In this sentence, again, more is an indefinite pronoun serving as the object of the verb earn:
I earn a lot more than my sister does.
Here, "a lot" is also not an adverb but a determinative phrase (DP) that modifies the indefinite pronoun more in the comparative phrase more than.