Please help little Nina get back to her old self

Hi, my name is Emma and I never thought I would be doing this but here we go. We got Nina as a pup a little over a year ago as our daughter was still heartbroken over the loss of our old boy Bobby. Not your typical Chihuahua, Nina was the most loving, sociable little girl you will ever meet. Full of life and licks, she would go to anyone for a cuddle. Safe to say we have all fallen in love with her and she has stolen our daughters heart. About 6 weeks ago Nina started to be lethargic, wasn't eating and was walking very cautiously and would yelp when picked up. She was quite obviously in a lot of pain and the vets think it's some sort of infection in her spine. After lots of trips to the vets, a couple of day admissions and overnight stays, emergency vets treatment, two ultrasound scans, numerous blood tests, blood and urine cultures, pain medications and antibiotics, and apart from knowing she has a constant high temperature we still have no answers. From the day she fell ill she still hasn't been able to jump up onto the sofa, come down the stairs or play with toys. She now cowers when people to go to her in fear of being hurt and is very introverted as a result. We had her insured from day one but unfortunately we are now way over the claim limit. I received a call today to say that Nina needs further investigations of either an MRI or CT scan along with more blood tests, another neurological examination and a lumbar puncture to find the cause of the illness and find the right treatment. If we don't do these tests, because the antibiotics haven't made much of improvement after weeks of treatment there won't be any other option and she will quite possibly keep getting worse, and worst case scenario, put to sleep.

It seems such a shame that such a young healthy dog has taken ill this soon and hate to think what our daughters reaction will be if anything happens to her. I'm just asking if anyone is in a position to help out it would be worth a try. Thank you for reading.