The narrator has established in several instances that the Bertram children, though they were taught to be well-bred, very severely lacking in the inner essentials department.

For instance: The politeness which [Julia] had been brought up to practise as a duty made it impossible for her to escape; while the want of that higher species of self–command, that just consideration of others, that knowledge of her own heart, that principle of right, which had not formed any essential part of her education, made her miserable under it. Chap. 9

Sir Thomas himself finally comes to understand this: He feared that principle, active principle, had been wanting; that they had never been properly taught to govern their inclinations and tempers by that sense of duty which can alone suffice. They had been instructed theoretically in their religion, but never required to bring it into daily practice. To be distinguished for elegance and accomplishments, the authorised object of their youth, could have had no useful influence that way, no moral effect on the mind. ...his cares had been directed to the understanding and manners, not the disposition; and of the necessity of self–denial and humility, he feared they had never heard from any lips that could profit them. Chap. 48

Finally, Sir Thomas comes to believe that struggling in life and persevering through hardships is more advantageous than having an easy life: Sir Thomas saw repeated, and for ever repeated, reason to rejoice in what he had done for them all, and acknowledge the advantages of early hardship and discipline, and the consciousness of being born to struggle and endure. Chap. 48

We don't get here of it in this particular set of chapters because this is not the appropriate context for it. We are seeing Portsmouth through Fanny's eyes. In fact, the ON warns us that Fanny is not entirely unbiased. However, even Fanny comes to reverse some of her harsh opinions on Susan, at least. It's never been JA's style to expound on her personal views extensively. She gives her thoughts, or rather, the narrator's thoughts, in very limited doses.

I do not agree that JA didn't make allowances for Mrs. Price's situation. Jane Austen says that Mrs. Norris would have been more respectable than Mrs. Price in the same situation, I don't see where the ON says she would have been a better mother to her children. Wherever she goes, her basic nature would be the same. Similarly, Mrs. Price was not born into a lower income. As the ward of her uncle, an attorney, she would have grown up as a gentle-woman or close, and would have had little or no knowledge on how to manage a family on a lower income. So, she brings these deficiencies into play in her current situation, and has not made much efforts to become better at it.