Is there a way to have the margins disabled in Card view?
Sure. I re-wrote all the renderers in the upcoming release, and still need to adjust the existing views to account for those changes. I can add a "Wide Card" view since I'll be working in that section anyway.
Comments button does not open in a new tab
That's fixed in the current dev branch (1.4.30). I noticed that myself not too long ago and fixed it yesterday.
When I am in a community...and click the plus button to add a post, the community field is empty
That was kind of an intentional choice on my part. The community sidebar button "Create Post" will pre-fill the community, and you can click the community name and chose "Create Post" with the same pre-fill behavior. My thought was that if I was in community A, and thought of something for community B, I should have some button available that would not pre-fill the community so I could select one.
That said, I agree with the point you're making. I'll see if there's a clean way to address both. The only hiccup is there's no way for the options in that button to know if the community is restricted to mods posted, so that may cause a soft bug.