(moderator please move to the Ecommerce Hosting board)

You'll get better repsonse on the board dedicated to e-commerce issues. My take on your questions:

1. It is always best to keep different applications or subsystems in their own directories. This keeps everything cleanly separated and out of each other's hair, and permits easy updating or removal. An online store would be kept in public_html/store/. To get to your store, if you have nothing else on your site you can just have a simple index.html file that includes <meta http-equiv="refresh" content="0; url=/store/index.php" />. If you want some sort of entry or splash page, you can write your own with a link to /store/index.php to get people to the store. There are more ways to do it, but those are the easiest to start with.

2. osCommerce (osC) is written in PHP, so I would just use a flat text editor such as vim (is there a Mac version?). Don't use Word or any other WYSIWYG editor -- it will leave hidden junk in your files. If your system is set up for UTF-8 character set, check to make sure your editor doesn't leave a "Byte Order Mark" at the beginning of edited files. There are probably some editors that highlight syntax in the code (vim can), but I don't think there are any WYSIWYG editors for PHP. That wouldn't make sense, as PHP is a dynamic programming language, rather than static markup for display (HTML).

3. See #1. You don't have to have visitors stop at a Home Page, especially if the only thing on your site is the shop. In that case, just redirect them directly into the shop. If you have other applications (gallery, etc.), a Home Page is good, to let them know what else there is to see.

4. It is difficult to have consistent look and feel when you're moving between different subsystems (applications). Each has its own design. Certainly it's possible to do major surgery to get them consistent, but by that point you might as well write your own applications. Usually it's not hard to set your own background color or use your logo and title in some place like osC, but beyond that, it becomes a lot of work (especially when it comes time to update the application, or add canned "mods" to it). You may want to consider adjusting your template (or hand coding) for your pages to somewhat match osC's look and feel.

5. osC is certainly very good, and widely used and well supported. Is it perfect? Of course not. You may find a different shopping cart that is a better match for what features you want and is easier to "bend" to the look and feel you want. Try out several, installing them in separate directories. They won't interfere with each other, and you get to play with them until you make a decision.

6. There are a number of "gallery" applications, both from cPanel > Fantastico Scripts and from sources on the Web. Just keep in mind, it's probably going to be difficult to integrate a display gallery with a shopping cart. Maybe you can find a gallery that offers an option ("mod") for a shopping cart. If you just want to display artwork, but not try to sell from the gallery, you might be able to install separate store and gallery software. Perhaps there is gallery software that permits you to (manually) add links to the sales pages in your store? Look around, and good luck!