In theory, yes that could be done by editing body.php. in this file you can just comment out the parts that you dont want. this should not change the way the forum works because all the *logic happens before the body.php and the body.php just takes the variables and builds the page with them.
I must say the forum has been made to run with all its parts and im not 100% sure what would happen if you ran it with just the comment part. it might work, but it has not been tested.
If you want to change the way the forum looks, you can easily do that by changing the HTML code in the body.php file and all the other files that have names starting with "bod_"
But if you want to make a simple comment box for your site for example, i would personally think it would be easier to just make one from scratch than using a whole forum system to make a simple comment box system. maybe then to do things clean, you could include the bbcode.php from this forum to support the bbcode in your comments and use the _nodesforum_bbcode_interface.js from this forum as well to spawn the bbcode button over your comment box. but then take care of writting/reading of the comments yourself in a simple database table instead of using the complex forum structure for nothing.
yes, you can run multiple instances of the forum on 1 same database but you need to make sure that the value $_nodesforum_db_table_name_modifier in the config.php is set to something different for each install and this way they will be independent forums.
so basically lets say you already have a nodesforum on your site and then you want to use a nodesforum somewhere else on your site to turn it into a comment box (which im not sure is the best idea) you can at least safely experiment with this as long as you make sure that the 2nd install of the forum, the one youll try to turn into a comment box has a different value on the $_nodesforum_db_table_name_modifier variable in the config.php file.
this way the 2 forum installs will have each their own tables, they will not interact with each other.