Can breast cancer be detected by ultrasound?

Yes, breast ultrasound can detect breast cancer. Reported sensitivities vary, but in general the overall sensitivity for detecting breast cancer ranges from 60-80%, and is similar to mammography.There is some data to suggest ultrasound may perform better than mammography in younger women and in women with dense breasts.
Sometimes yes. But diagnostic tests are never 100%. False negatives and false positives occur with some frequency. This is also true with mammogram and MRI. Breast ultrasound is nowhere near 100% for cancer diagnosis. Definitive diagnosis is always tissue biopsy.