Very interesting question! You've released the proverbial elephant into the low-VOC, energy efficient, sustainably constructed room.
Clearly, you are trying to make a decision on products that you offer through research and opinions from the field. I applaud your green consciousness on the topic. While I am only one voice on the topic, hopefully I can offer a valuable opinion on the matter.
Let's start by discussing some basics about PVC: The manufacturing process is toxic and the product releases additional toxins if burned in municipal or household waste streams. These facts are well documented and are the basis for the argument that PVC should be avoided and even boycotted during the decision process when building or renovating green.
I stood on the sideline for a long time on the subject of vinyl products used in the home building and renovation process. With the health issues of manufacturing and disposal constantly battling durability and maintenance issues in my mind I struggled to make the "is it green?" decision. Durability is a very big factor in my definition of green that is equivalent to making a decision about health in the home.
One day I walked through a basement of a LEED Platinum home and I realized the staggering amount of PVC used without a thought. It was part of the heating, plumbing, electrical, and common systems throughout the house. After all, the product is chemically and biologically inert and several of the arguments made in your question make it extremely useful. This usefulness may be the reason that PVC pipes account for a majority of the plumbing market for both water distribution and wastewater in the United States. Most estimates say that the market is growing!
Yet the question of outdoor installation of siding and railings, etc., is heavily debated because there are alternatives available that have a lower impact on the environment right now.
While I will agree that there are alternatives to exterior PVC installation right now, I haven't seen anyone demonstrate the shade of green over the lifetime that includes maintenance on the alternative products. Does the manufacturing and disposal of paints, stains, and sealers over the lifetime of these alternative products not have an impact?
This is the point where I made my decision. If one cannot provide a lifetime comparison of two products and demonstrate superior attributes with regard to health and there is no immediate hazard being introduced into the home, then the durability factor has to take precedence.
Additionally, there has been a remarkable amount of news regarding the progress of recycling PVC and even research into lowering the manufacturing impact on the environment. I'm hopeful that someday the residential use of PVC will be comparable to the commercial use of steel.
Ironically, steel is considered universally green because any steel purchased today includes up to 99% recycled product despite the extreme CO2 emissions produced in the process. Each person has their own definition of green.
Unfortunately, there are no facts to support a broad statement that PVC is green. However, given the outdoor use of a PVC product that contains a high percentage of recycled content, I would argue that based on durability and recycled content you are offering a green alternative to what is available on the current market.