A) The 5.2 update is planned for Joomla 4.3, not out yet. While we will update at some point, we won't get in front of Joomla. Additionally, unless there's a direct conflict or bug we're made aware of, we won't update immediately due to the amount of testing and knock-on issues that can be caused from updates.
If you look at this file and search for .btn, it will be around the 10th result:
C) I'm not sure what Melissa was going for above either. Generally, .btn styles should have a 'type' also used, like primary, warning, danger, etc.. like:
<input class="btn btn-primary">...</input>
The .btn class *is* transparent and the btn-primary sets the color.
The btn-login is what we use to set that color. However, on your site, your template seems to be inlining (including in the HTML) the btn-login colors. I think there's an issue with the order of the bootstrap.css file and that inline code which is causing the btn-login colors to be ignored unless they are set to important.
I hope that helps explain some more.