Yes, you can either upload the files yourself and install by pointing your browser to the root directory, or you can use cPanel or a similar tool that your web host provides to install phpBB3. To prepare your forum for Search Engine Optimization, you need to install the extension located here - Ultimate SEO Friendly URL for phpBB3. Install it as you would any other extension, by uploading to the /ext/ folder in the root of your phpBB installation. Inside the /ext/ folder you need to create two more folders so you can eventually upload the extension to <subdomain.yourmaindomain.whatever>/ext/phpbbseo/usu/<extract and upload everything inside the usu-master.zip file here>
I find it easier to do that so you don't have to mess with virtual folders.
Yes, Wordpress makes more than one plugin for that. I've never done it, but you may have to investigate and try a couple out. The one below has the highest rating.
WP w3all phpBB plugin for WordPress
And if you want to search for more, click here - Search for WordPress extensions for phpBB.