This is a bit of a weird passage.
You said:
"I'm confused whether the author agrees with Caroll or not...(or agrees with Ashenfelter or both.)"
The author never explicitly agrees or disagrees with either, but he seems to implicitly agree with both.
When I read line 30-32, it felt like the author takes Carroll's notion very seriously, seriously enough to talk about the possible ramifications of Carroll's notion, IF that notion is true. The author does make a point in line 32 of saying "
if right-to-work laws lower wages by weakening union power".
You wrote:
"Caroll does not explain how they impede the spread of unions and how that reduces wages in the union, does s/he? Caroll just argues they do...
(Also does not explain why the countervailing power of unions is weakened but argues it is in line23-29)
When a passage does not explain reasons, should we just accept what it says? (or maybe just me overlooked them in this passage?)"
I don't think we're supposed to blindly accept that Carroll is correct. The author is still leaving room, in line 32, for the possibility that Carroll's idea could be wrong.
And I agree, we don't really get any reasons for why right-to-work laws impede the spread of unions. However, given the explanation of right-to-work laws, which allow states to prohibit a certain type of union agreement, we can probably fill in the blanks on our own as to how right-to-work laws are leverage
against unions.
If you accept the idea that unions have less power in these states, then the idea in 25-29 sorta just follows. The power tension in labor is between industry and unions, so if unions have less power, industry has more. The more power unions have, the higher wages would be. The more power industry has, the less wages would be. This passage somewhat demands that we get that common sense relationship of who's fighting for what.
You wrote:
"Also...when the passage says "there is positive impact" (line 38) it does not mean there are good impacts, right? Just means there is an impact...since it provides a reason to "minority workers can be expected to suffer a relatively greater economic disadvantage"...correct?

"
No, I think 'positive impact' here does mean 'good impact', at least from the perspective of Black workers. Unions tend to get Black workers better wages, and unions tend to reduce the disparity between White workers' wages and Black workers'.
The quote you gave is not talking about the effect of unions, it's talking about the effect of right-to-work laws.
Unions = good for Black workers
Right-to-work laws = bad for unions
so, Right-to-work laws = bad for Black workers
You wrote:
"So the first paragraph is introduction introducing the law, how some literature responds to it and how one specific person responds to the literature's responce, 2nd paragraph is about the implication of the responcce and introduces another who points out insufficiency of the person's argument...(not necessarily disagree with the person's implication?)"
Most of this is SPOT ON! But Ashenfelter isn't finding any fault with Carroll's argument. Ashenfelter is finding fault with the unnamed people from line 37 ... "what was once thought" and the unnamed people from line 41 ... "Most studies concerned with the impact of unionism".
Carroll is in this passage to bring us this idea:
Right-to-work laws = bad for unions
Ashenfelter is in this passage to bring us this idea:
Unions = good for Black workers
The author is trying to bring us this idea:
so, Right-to-work laws = bad for Black workers
Hope this helps.