Hello Thom.
I will try to answer some of your questions.
First, the leaking evaporator is not such an uncommon issue. I know of at least 3 Boxsters that have that issue (including mine).
First take a look at this diagram:
Here you can see the location of all the O-Rings in the system.
Porsche Boxster Refrigerant Circuit Parts
If to give a direct answer to your first question, there are 4 O-Rings between the evaporator and the expansion valve (2 from each side of the valve). The O-Ring can be reached from the passenger side of the front trunk.
In order to get to it, you will have to remove the nut (Red) and the retaining plate (yellow)
Now you will have to remove the two Allen screws to get to the other side of the valve.
This is how it looks with the valve removed. In this case the air box is out of the car, but it can definitely be done when the air box/evaporator still in the car.
With that said, I will find it hard to believe this is your problem.
When I removed the O-rings in my car, they were in perfect condition (after 12 years of use).
If you filled the system with AC dye, it will show pretty clear around the valve (if the leak is there).
It is very easy to detect if the AC evaporator is leaking.
Assuming you still have dye in your system let the AC work for 5-10 minutes when the car is parked.
A puddle of water will show under the car. This “water” is the humidity in the air, accumulating on the evaporator, and dripping down to the bottom of the air box. From there it exits the car thru a draining tube.
If the evaporator is leaking, the water should be contaminated with the dye, which can be detected by using a “black light” flashlight. Another good sign for evaporator leak is the smell of a “cat pee” coming out of the air vents. (I never owned a cat so I’m not sure about that, but that what other people told me)
For the second question.
I’m not sure if the “stop leak” is a good idea. I don’t know if it will stop your leak, and how it will affect the system on the long run. I have to admit that this option has crossed my mind as well.
But after thinking about it, and knowing (or hoping) I’m going to keep the car for a long time, I toke advantage of a rainy weekend and came up with this:
AC question
As for the last question, I do not know. I toke the car to the AC guy and he evacuated and recharged the system based on his knowledge and the information he has in his data base.
The all process cost me less then 400$, and I even enjoyed working on my car (like most of the time)
I will be happy to answer any other question you might have.
Good luck.