Hi EJ, I will try to answer you questions by pasting in your questions and responding to them individually.
Q-Down to what temperature F can you comfortably use the shell with the PL-400 glove while still or moving? A- While still maybe somewhere in the teens; they're warm but not that warm. While moving they would be warm maybe down to zero. It depends a lot on the person and how dry the liners are.
Q-Do you find the Endeavor/Latitude breathable enough? A 2007 Backpackgeartest.org reviewer at
http://www.backpackgeartest.org/reviews/Clothing/Gloves/Outdoor%20Research%20Latitude%20Mitt/Test%20Report%20by%20Sheila%20Morrissey/ noted as a con that “The mitt shells are completely waterproof - awful for skiing as my hands were drenched in sweat!” Sweat rate differs from person to person, but just wondering how well the shell mitts breath when you are very active, such as digging a snow kitchen or building an igloo. A- Like most GTX products, they are not very breathable,and moisture from sweat builds up in them. A very small amount of moisture will make your hands very cold. That's why I recommend taking several pairs of liners along so you can keep changing them.
Q-Is the Latitude cut true to size like the Endeavor? A- The Latitude is the same shell plus a liner, so they are cut the same.
Q-If you size true to size, would there be enough room inside the PL-400 glove to fit a thin liner glove like the Ibex liner glove or our light powerstretch glove? A- It depends on your actual hand size of course, but the PL-400 mitts I have are large enough to add a liner. I recommend the mitts instead of gloves because they are warmer. I prefer to wear a glove liner inside a mitt.
Q-If you oversize 1 size, do you lose a lot of dexterity with the larger shell mitt, especially when using a thin liner or no liner underneath in milder weather? A- Yes, with a shell mitt you do lose a lot of dexterity. Shell gloves have a little more dexterity but they are not as warm, and the seams in the fingers come apart. I prefer using a shell mitt and have a wide variety of liners to wear inside.
Q-If the Lattitude is true to size, and you size up for winter, won’t the included PL-400 gloves be too big for your hands and less dexterous? A- Again it depends on your actual hand size. I size up to XL and the mitt fits a little loose, which I like. Mitts don't have much dexterity no matter what size.
Q-If we don’t already have a heavier warmer fleece glove like the PL-400, do you recommend it over similar ones? Is there a similar weight fleece/softshell glove that you like better? A- My warmest fleece gloves are some no-name gloves I picket up at a sale somewhere. The PL-400 is warm, but not that warm. I would simple look around for really light, thick fleece gloves, like Turtle Fur. You pretty much have to look at them at a store to find what you want because a description online does not really tell you how thick or how warm they are. I have 2 bushel-size boxes full of shells and liners that I have accumulated over the years.
Q-Do you find it necessary to renew the DWR on the surface of the mitt once/season with regular use? Which waterproofing spray would you use to keep them waterproof but breathable? A- I have not done it yet with the Endeavor Mitt, but I would use Nikwax Tech Spray.
Hope this helps out-