bbPress 2 is in it’s very first release (about to be second with 2.1).
There is only so much functionality that can go in per release.
Can’t say I really understand what you are asking for, but if you are looking for shortcodes
https://bbpress.org/forums/topic/bbpress-20-shortcodes
Without the URL I can’t really say. I can’t inspect the CSS from a screenshot.
What version did you install?
I’m not sure I understand? You want to run both versions at the same time?
Temporarily switch you theme to TwentyEleven and see if that fixes the problem. That will rule out your theme being the cause.
If it works with TwentyEleven then unfortunately it’s an issue with you theme (typically the theme is doing something strange and not orthodox).
I’m not familiar with the Arras theme, I would contact the theme author and see if they have any insight.
Without having the theme it’s hard to tell, it could literally be 100s of things.
I noticed the theme is no longer available in the WordPress.org theme repository, looks like it got pulled. That’s a little concerning as themes are pulled from the repo when they are either not written correctly or doing something shady (putting links in your footer, etc).
You are using 2.1, which is good.
What you need to do is tweak the CSS, I don’t recommend editing core, I would use this instead https://wordpress.org/extend/plugins/bbpress-custom-css-file/
bbpress.css, line 48 is what you need to change, ie
#bbpress-forums .status-closed, #bbpress-forums .status-closed a {
color: #CCC;
}
You marked this as resolved is seems, did you get it figured out?
If you are using bbPress 2.x, bbPress stuff does not get put in the primary RSS feed by default. If it is then I would check and see if maybe a plugin is screwing something up.
If you are using bbPress 2.x it has widgets that are packed with it out-of-the-box that provide these functions.
If you switch your theme to TwentyEleven does it still happen?
I’m pretty sure this is an extremely rare but known bug. Let me see if I can get jjj’s 2 cents on this. Stand by!
Without the URL I can’t test out anything, hence I can’t answer your question.
This is what’s messing it up (from what I can tell)
div.bbp-breadcrumb {
float: left;
}
You need to change that to float:none;
Without the URL I can’t really tell 
No you shouldn’t have to do any of that for 2.0.x or 2.1.
Does the shortcode show up on the page or just nothing?
BuddyPress is entirely modular, so you only have to activate the parts of it you will use (such as PM) 
You may want to looking into BuddyPress, it can handle groups and such natively
I’m sure something surface eventually, however as you mentioned at the moment there isn’t a working solution for bbP 2.x.
The thing is that the majority of bbP 2.x users (dont have any raw numbers) are using it in conjunction with BuddyPress… so they already have this feature.
Most people who want private messaging also want extended user profiles, etc.. which just leads then to BuddyPress.
Have you launched this site (meaning, is it being used already) or is it still in development?
You should be able to just put a few CSS tweaks/overrides in your themes’s style.css.
Are you familiar with CSS?
As I said in the other threads you bumped (https://bbpress.org/forums/topic/unread-poststopics-plugin-or-workaround-for-bbpress-20)
Currently there is no plugin that handles this (at the moment) for bbPress 2.x.
Anyone can write a plugin and contribute it to the WordPress.org plugin repo.
