Nunam Iqua,
I have a WM bag of the version that preceded the Versalite, also rated at 10 degrees, also weighing around 2 pounds, also too thick for the summer!
I have section hiked from GA to just past the Hudson River, most of it in the off season in relatively cold conditions. For this, a Versalite bag was appropriate as I knew I would only be out for two weeks and wouldn’t be stuck with a useless bag in hot weather. Section hiking gave me the opportunity to experiment with different options (bag, pad, shelter and cooking) to find what works best in what conditions. Having qualified myself, here is what I would recommend you do given your desire to buy only one bag:
Buy a WM 20 degree bag. WM has a reputation for understating their temperature ratings unlike other manufacturers. Get the best down you can afford.
Buy your WM or equivalent ultra-light weight jacket, hooded or not. Use this jacket to extend the range of your bag by stuffing your feet in the jacket. That’s right, your feet!
Buy an insulated (closed cell foam or Big Agnes insulated inflatable) sleeping mat. Stay away from self inflating mats as they are not closed cell foam and don’t insulate well (as I found out one very cold night on Mt. Rogers).
When it gets real cold out, sleep on the ground, not in a shelter. The floor boards in a shelter will cool off to the air temperature whereas the ground will not.
Finally, use hand and feet warmers! They are available everywhere, they are cheap, they get lighter as you go and they are effective! Plan on two pairs of feet warmers per very cold night. Use the hand warmers during the days in your gloves or pockets. They will make a big difference in your comfort for very little weight gain and very little cost and, when the weather turns warm, you can drop your excess in any hiker box to get rid of the weight.
And, that’s how I now go hiking down to the upper teens in a 35 degree WM Ultralite bag!
Good luck and remember to HAVE FUN!!!
Bob
Bob Martin