Hi hope!
It's now possible through DNA sequencing machines to explore the entire genome of a person. It allows to detect specific mutations on genes known to induce certain diseases. There is a margin for error in the tests.
The test is not 100% accurate because even if the results reveal the presence of specific markers (in this case related to UC) it doesn't mean those genes are responsible for your daughter's condition. For example, if you got yourself tested and they found markers for a mutated gene known to cause Alzheimer, it doesn't mean you will actually get Alzheimer.
From what I gathered, the only way to differentiate UC vs Crohn's is through histology, aka studying the cells under the microscope. The doc takes a piece of the bowel during a colonoscopy and sends the biopsy to the lab. They stain it and look at it under a scope. That way they can tell whether the colitis is acute or chronic, active or inactive, etc. The cellular activity is different for UC and Crohn's and a pathologist will be able to tell.
As for your other question, both UC and Crohn's can evolve in patches. The biggest difference between UC and Crohn's is that UC will never touch anything outside the intestine, whereas Crohn's can involve the entire GI tract, from the mouth all the way to the anus. This gives UC patients an ultimate option for a "cure": they have can their bowel removed and get rid of the disease altogether, giving up all the meds and treatments. As sickinlk said, there are some people who have a bad case and for whom a return to normal life is worth any price. Crohn's patients do not have that last resort.
Aside from that, biologically speaking, UC and Crohn's are extremely similar in how they work, and how they are treated.