If she hasn't received a proper diagnosis for her sudden weight loss, and is now on the proper treatment to help her get better, or at least stabilized, then you need to get her in sooner than 2 weeks, as an emergency visit if necessary.
She is extremely thin, to the point that I consider it potentially life threatening. Adding in high calorie food isn't going to make that much of a difference with how severe her condition is, if she hasn't been diagnosed correctly and is on the proper treatment.
So if this hasn't all been done already, the most essential thing at this point is getting the proper tests done to determine the cause of the weight loss and getting the proper treatment started. If these tests haven't been done then these are the diagnostics I would expect. A thorough dental check to make sure that's not causing eating problems and the weight loss. If the teeth are ok, a fecal float test to rule out parasites and a blood test to check liver and kidney function, and for signs of an infection. If neither of those show any problems, checking the heart, possibly an ultrasound of it. If nothing is wrong there, xrays to see if there isn't something else going on internally, maybe even cancer. And possibly a blood test for e. cuniculi.
In the meantime, you can add or increase other foods if you want, but I wouldn't add alfalfa in case it's kidney problems(high calcium isn't good), unless a blood test has already been done and shows kidney function is fine. I would just increase pellets and maybe try oats. But the most essential thing is getting a proper diagnosis, even if you have to take your rabbit to a rabbit specialist(if you haven't already).
I can't recall if you've ever mentioned what vet you use, but if you aren't seeing a specialist or at least a really experienced rabbit vet, Beaumont animal hospital and Trinity Vets might be two possibilities somewhat close to your location. Or if you aren't able to provide the necessary medical treatment and there's a rescue that will take her, then yes, surrendering her to a reputable rescue might be the better choice.