1. If you are setting up an LLC, I think you need a separate checking account (don't quote me on that, I said "think").
3. Do you live in the US? If yes, probably couldn't hurt. Profits earned through the LLC should be taxed as a business, which is a lower tax rate than personal. However, I do not think that money is 100% accessible for you as the owner without "paying yourself a salary" on behalf of the company. A salary which, to my understanding, gets taxed like any other salary. So there is the potential of "double-taxation", but the combined tax might end up lower if you consider all the perks a business can take advantage of (such as business-expense-related write-offs and whatnot). For example, Google and Apple charge a dev fee to put your app in their app stores (Google is one time fee, but I think Apple charges annually). But, with an LLC, you could probably write those fees off in your taxes (you'll have to check with a tax consultant).
**Make sure to confirm EVERYTHING I've said with a tax consultant**
4. For mobile apps, create two versions: free and paid. The free version should have ads. The paid version should be ad-free and possibility contain more functionality. Having both allows you to capture a larger portion of the market. Just having a paid version means you lose ad revenue from cheap people (like myself), and can also mean you lose the portion of the market who wants to "demo" an app before purchasing it.
5. Depends entirely on the type of apps you are making. In general, it doesn't hurt to have a website promoting your "brand".
Edit on my tax comment: actually, I don't think the LLC does pay taxes: http://www.irs.gov/Businesses/Small-...pany-%28LLC%29
Like I said, the best way to check is with a proper tax consultant.