Hello,
1) If you wanted to remove the button styling you would have to remove the button class call in the coding:
<a class="btn" href="#">VOIR PLUS</a>
The .btn class is what generates the padding and background for the button.
2) In regards to the alignment I would check your coding in each of the separate accordion modules. In the page source I notice you have additional <p> tags on certain elements that would affect spacing on the site that aren't evident in the other modules.