Nelle your right the cannula is just for precautions.

Where I work most midwives do not put cannulas in (a few do, and out of them not many are experienced so put them under pressure in an emergency they are just as likely to miss as to get it in) to call a Dr they would come from home or from ED so it would be a 5min wait in an emergency at least to get the Dr to put it in (if a midwife couldn't do it) out of our 25midwives about 5 cannulate so not always is there a midwife on that will be able to even attempt. Some places have Dr's there all the time so this wouldn't be a problem but many places such as where I work do not have drs but one or two on call for emergencies.

Yes a hospital would be protected if the woman signed something to say she was happy to take responsibility for no cannula or leaving against medical advise for example.