I also think you're on the right track. EMS makes good tents, and nearly any lightweight, 3-season one-person tent will work fine for Whitney.
For a sleeping bag a 20 degree down bag is probably the most versatile. If it's too warm for some nights, unzip it and use it to cover you partially. To add additional warmth, consider bringing 2 pads - a self-inflatable one plus a closed foam. They can be 3/4 length. You'll be more comfortable plus warmer.
Before going to sleep, make sure your clothing is completely dry. And have a snack - it helps crank up the metabolism.
Bring a warm hat, one with earflaps. I like wool hats, lined with microfleece, but there are lots of choices. Bring warm mittens - not gloves. Mittens are much warmer. A double-layer fleece is a good choice if you can find them. Consider wearing the hat to bed if you run cold. I also like a neck gaitor like a buff - they're very lightweight, and can be used in a variety of configurations, including a face protector.
Finally, bring a waterproof, breathable shell with a built-in hood. You don't have to spend a fortune of them - LL Bean makes a quite suitable one for about $80.
