Is there a table that summarizes the changes that were introduced by the 2012 version of the standard, as compared to 2003?

No, not exactly. Because the CPG 2003 was not an immediate predecessor to 2012, there is no resource available that explicitly compares the two. However, there are resources online that compare the 2008 iteration to the 2012 CPG, like this page.

If a building only has one of the required systems, is it enough to submeter that one or are you not eligible for the credit at all?

If you have absolutely no other uses, then USGBC's intent is that the project is not eligible for the credit because it's not doing anything beyond what's required in the prerequisite. However, there are often items that can be treated as either domestic hot water or processes. Note that if the water is going to two uses, you can use one whole-building meter and one sub-meter to derive the usage of the second process by subtraction.

Are there any requirements for the frequency of meter reading/recording?

While metering duration and frequency are not specified for this credit, daily and weekly readings provide much higher resolution and better feedback to building operators, compared to monthly and annual tracking. Try to integrate frequent water use reading and analysis into standard operating and maintenance procedures.  

In the past we've been able to get credit for buildings contaminated with asbestos. Is that still true?

No. Asbestos and other contaminants inside buildings (whether demolished or remaining) or in debris do not count as contamination under this credit requirement. USGBC's intent here was to exclude building materials, unless components of the old buildings are found in the soil or have leached into the soil/water. Asbestos in buildings is usually required to be addressed by law, or is something that is pretty commonly remediated now, and thus isn’t something that projects get points for under LEED.

The priority designation boundary where our project is located is set to change soon. Will we still qualify for this credit?

This is a new LEED credit and we haven’t seen a lot of precedent for this situation. In our opinion, GBCI is most likely to reward the credit based on the status of the designation as of your LEED application date, but in other circumstances you could check with GBCI and make your case.

Where do we measure the radius from? In LEED 2009, we measured it from the main entry, but the LEED v4 language is different.

The density radius is not from a main entry or from the center of the project site. It's based upon an offset from the LEED project boundary. Because of this, it will likely end up being a weird shape, rather than a typical circle. Also note that measurement to basic services are based upon walking distance rather than a radius.

When it comes to satisfying the requirements for Option 1, what does the "total buildable land" take into account? And the "total building area"?

“Total buildable land” represents all land within the ¼-mile radius that can be built on. This does not include streets, sidewalks, or any area that is not available for development.

“Total building area” would account for all types of buildings and homes within the ¼ -mile radius, if the project is pursuing combined density. Note that parking garages are excluded from this.

The owner has purchased RECs for a percentage of energy use for their entire building portfolio. Can we use the purchase to earn this credit for a single LEED building?

Yes. Provide evidence of the quantity and term purchase for the portfolio along with an explanation of how the green power has been or will be allocated as applicable. If any of this purchase has been allocated to a previous LEED project, state how much and provide a letter allocating the quantity needed to this project.