- Raj never looked cross or harried. Also, Raj never looked plump. In comparison, she had become plump after the first baby.
alternately
- Raj never looked cross or harried. Also, Raj never looked plump. Meanwhile, she had become plump after the first baby.
There is nothing in this passage which suggests to me that one interpretation of the subordinating conjunction should be preferred over any other. The meaning of "as" in this context seems practically irrelevant. What is relevant is that it relates a subordinate fact (she had become that after the first baby) to a complement of the matrix clause (plump, which is something that Raj never looked).
The problem with A) is that "had she become" uses a word order that we reserve for questions and subordinate subjunctive clauses -- neither of which is a fact. An element that is already subordinate doesn't need a subordinating conjunction. An element that isn't a fact doesn't make sense with the conjunction "as".
The problem with B) is much smaller. I wouldn't even call it an error. It's merely redundant.
- Raj never looked cross or harried, or plump as she had become [plump] after the first baby.
You can consider the repetition of the complement "plump" to be grammatically present, even though it is omitted as an unnecessary and confusing repetition.