Some Questions about Blog Layout - Please Help

Support Discourse Landing Pages
1

So, if I choose Blog or Blog Post, category stays greyed out. I have 2 categories on my site: Forums and now Blog. Was I supposed to make Blog a sub-category with Forum as the parent?
yyyy

Still a little confused on the theme thing. Is my entire forums site supposed to be Blog Landing theme, or only on the plugin? Like, if I want to use my modified version of Air, can I keep Air as my forums theme and then just set Blog Landing theme in plugin settings? I’d hate to have to re-make my entire theme.

I also can’t seem to figure out how to make “forums.mysite.com” the landing page. I would like if people got to the landing page by visiting my forums URL. I also can’t change my path at all?
https://i.ibb.co/j87712R/kkkk.png

Lastly (for now), how do I make the blog landing page URL the default page people get to when they visit my forums?

3

Here are the main issues in a video:
Subscribe info is weirdly in bottom-left corner with a super long Submit button at very bottom of screen. Subscribe button on bottom-right does nothing. Has awkward checkbox next to it. Finally, an invisible box that glows when hovered over that does nothing, but is clickable.
https://i.imgur.com/1RUAsRP.mp4

Also, are there supposed to be all these errors in the HTML code? Because I suspect that’s why the Subscribe area looks all jacked up at the bottom.
https://i.imgur.com/GAmom0O.png .