Is it possible point a second Drupal site to an existing Drupal database, if that second site is only used to browse the content, no logins, registrations or content add/edit/deletes?
Is it possible? Yes. Should you do it? Probably not.
Using table prefixes, you can run two completely separate sites with table names like "site1_node" and "site2_node", so they don't collide. In your case, it sounds like maybe you also want to share data between the sites?
On solution to that, is to share tables between these two, but I strongly recommend against that. I've tried it on two different sites, and I've mostly gotten bit badly by it. Use Domain instead, to run two domains from a single Drupal install.
This is perfectly possible. Look for instructions about multisite installations combined with the domain module. Both sites can have common and separate settings, and both sites can have write access.
To set-up everything, you also need to configure your DNS settings and your server settings appropriately.
Yes, you are able to maintain multiple sites with single drupal installation. Please check this article: How to install multiple domains in a single Drupal installation.