Facebook – Username for Facebook app profile page

Depends what you mean by post to walls. If you really want if showing on their wall so that they and all their friends see it then yes you need an application. This might be annoying to all your users friends and they can block you. If you just have a page updates you post to the page will show up on the home screen (not profile) of your fans. Remember not everything on facebook shows up it's like SEO you have to do FBO

No, once you have created a profile on a business page it turns in to a personal page. This action cannot be reverted.

From this article on the Facebook Help Centre:

What happens if I click the 'Create Your Profile' button at the top of my business account?

If you click on the "Create Your Profile" button, you will be asked to provide additional information to allow you to transform your business account into a personal account. You will be required to enter your full name, a security question, and an answer to your security question. Once this final registration step is completed, these changes cannot be reverted. You will then permanently have a personal account on the site.

And from this article

If I turn my business account into a user profile, can that action be reverted?

No, once you provide the final registration information for creating a user profile, you cannot revert back to your business account.

If you convert your business account into a user profile, you will still be able to manage all the Pages and Facebook Ads you’ve created by clicking the Ads and Pages link in the left hand navigation menu entitled 'Applications'. The fans of your Pages will not have visibility or access to your personal profile. Any actions that you take as a Page administrator on your Page will show the Page’s name as the actor and not your personal name. However, if you wish to restrict your privacy so that other Facebook members cannot find you in search or add you as a friend, you can do so by clicking on "privacy" at the top any Facebook page.