I have all my zendphoto files located in the following directory:

/files/zenphoto/

I keep most libraries of large code in /files/. I created a mod_rewrite as such:

RewriteCond %{REQUEST_URI} ^/photo_gallery/(.*)$
RewriteRule ^(.+) "/files/zenphoto/%1"

and then in /files/zenphoto/index.php
I changed
Line 15: define('OFFSET_PATH', 0);
to:
define('OFFSET_PATH', 5);
$const_webpath = '/photo_gallery';

Is this the best way access zendphoto things as /photo_gallery/?

it seems to work so far, but I want to make sure I am not breaking anything on the backend

Just to note it seems the OFFSET_PATH is used quite often, I saw that full-image calls functions-basic.php for it, so instead I just did

define('WEBPATH', '/photo_gallery');

at the top of functions-basic.php until someone recommends a better way.

You can name the folder your install Zenphoto in as you wish, it does not need to be named "zenphoto". If you change the name after installing make sure to re-run setup so the htaccess file knows about the change. Actually you don't even need to install it in a folder at all. You can install it also directly in the root of your website.

Btw, you should never hack any core files (except custom themes or plugins).