No, marketing designations are improper as titles and should not be used.

The preamble is entirely optional.  If it is included, it should state the name of the applicant, the title of the design, and a brief description of the nature and intended use of the article in which the design is embodied. All information contained in the preamble will be printed on the patent should the claimed design be deemed patentable.

Example:

"I, John Doe, have invented a new design for a jewelry cabinet, as set forth in the following specification. The claimed jewelry cabinet is used to store jewelry and could sit on a bureau."

No, a design cover letter is entirely optional.  In the overwhelming majority of cases, a design cover letter is  unnecessary.

No, a design patent application cannot claim priority to a provisional application.

Here are eight such benefits:

1.) It creates a rebuttable presumption that you are the owner of the mark (note, however, that even with this presumption – someone else can still prove they are the rightful owner because they used the mark before you). 

2.) It gives you a legal presumption of the exclusive right to use your mark nationwide with the goods and services identified in your registration (while common law protection is limited to the geographic area where the business activity occurs and possibly to geographic areas where the business could reasonably expand).

3.) It puts the public on notice that you are the owner of the mark. 

4.) Your business will be listed in the USPTO database.  This means that others who are considering which mark to use for their own business may find your mark there, and it thus may prevent a confusingly similar mark from registering.

5.)  It gives you the ability to record your trademark with U.S. Customs and Border protection.  That agency will prevent importation of infringing or counterfeit foreign goods.  (You need to separately register for that service, however).

6.) It gives you the right to bring a trademark lawsuit in federal court (which is often preferable than bringing it in state court—whereas common law trademark owners must bring their actions in state court). 

7.) It gives you a basis for filing trademark registrations in many foreign countries.

8.) You can use the R symbol - ®.  (You can’t do that unless it your mark registered--although common law trademarks can still use the “TM” symbol).

Yes.  One solution is to take is a graduated approach from the business side.  You could begin your business by ensuring that you only sell goods or services in one specific class, and then on your trademark registration application, you could specify that one class.  However, it will be incumbent upon you to police your business to ensure that only those goods/services are being sold.  Several years down the road, when you want to expand your business and offer goods or services in another class (or classes), you could file another trademark application specifying the additional class or classes.

There is a risk with this approach, however.  The risk is that someone else could register a mark that is the same or similar to your mark, but in one of the classes where you have not yet registered.  Thus, not only could you could be foreclosed from using your mark to identify goods/services from that class, but this other party could potentially bring a legal action against you if you did use your mark to identify goods/services in that class.  

In your trademark registration application, if you claim that your business sells goods/services in a certain class, when it actually doesn't, you could wind up losing all rights in your trademark.

Similarly, if your business sells goods/services in multiple classes, and not all of those classes are specified on your registration application--that's another way you could wind up losing all rights in your trademark.
 
Some people believe they can “cut corners” on government fees by failing to mention all classes of goods/services that their business sells on the registration application.  This approach is almost always a bad idea.  You really don't want to spend dozens of years in marketing and advertising--building up your brand, and then suddenly lose all rights in your trademark.  For many companies, their trademark is their most valuable asset.