BuddyPress.org

1. It should work as the posts are WP functions

2. You would be adding the codes as seen here https://wordpress.org/extend/plugins/content-and-excerpt-word-limit/installation/ into your active theme. Find instances of the_excerpt and the_content in your active theme’s files and replace that with code given above after you’ve activated the plugin.
https://codex.wordpress.org/Function_Reference/the_content
https://codex.wordpress.org/Function_Reference/the_excerpt

3. No, this would not work in BP activity wall.