I want to add a few more thoughts:
I like recon, so here's a link to the webcams you might want to use remotely in order to keep tabs on the real-time conditions as your dates get closer. Yeah, it's pretty snowy.
http://www.yosemiteconservancy.org/webc ... aQod01ILJgI also have a couple more suggestions on where to propose. The first is the Snow Creek trail. Same aspect, elevation, and rough climb up, but camping is right at the top of the trail on the flats, and the view from where you're legally allowed to camp is phenomenal, as opposed to where you're going to need to be at the top of Yosemite Falls. You'll likely have more privacy too. It's also a primary cross-country ski route up from the Valley to Tuolumne Meadows and high country destinations, so the trail should be fairly well broken unless there's fresh snowfall. She'll never forget, I promise.
Another is, if you can't do the top of the falls, what about the bridge below it? You'll have more people, but it's close to being an almost equally profound spot, and you'll probably not only make her day, but the days of lots of others. That's kind of cool in and of itself. I doubt she'll forget that either.
If you do get to the top of the falls, please stay out of that channel for getting water anywhere near the brink. It's slick, it's narrow, it's roaring, and, it's certain death if you go in. Yosemite is incredible, but joy quickly becomes tragedy for too many every year where water is concerned. You're in the process of making life changing decisions, this shouldn't be one of them.
As far as winter gear goes- don't buy a ton of it, get only what's absolutely necessary and layer up with what you have, and bring a change. If your bags aren't rated to the conditions, supplement them with a couple cheap fleece blankets and maybe 4-season ground pads. If you have a good tent, bring the guy lines for it and use them if it might snow or you get winds. If not, get yourself a slightly heavier little bomber of a 3-season tent with good geometry for around $200 and you'll be surprised by how functional and cozy it can be.