That would be total folly. For some things, like blood type, certainly. Do not take that at face value. But I highly doubt anyone with half a brain would ignore medical information on a patient saying "I am severely allergic to drug XYZ" and turn around and give them drug XYZ explaining that away as "Well, the allergy alert info I found on the patient was not official". It would take a special kind of stupid to proceed along those lines.
I don't feel competent to comment on that. What seems logical or folly to me is largely irrelevant given that I'm no doctor or paramedic myself. I am pretty sure though that paramedics and ER personnel with many years of experience know what they're doing. I didn't get the impression that a medical info card would be ignored entirely, just that it wouldn't be taken at face value and basically wouldn't make much difference one way or another.
I suppose the pros have a system in place to address all the major points when treating an unresponsive patient. The way Jeff puts it here seems to make a lot of sense, pretty much confirming the above:
Keep in mind, however, that the presence or absence of medical alert information is probably not going to alter the assessment or treatment a patient receives from paramedics or ER staff. For example, all unresponsive patients, not just known diabetics, are going to have their glucose checked as a matter of course. Also, we know that diabetics also have strokes, heart attacks, drug overdoses, etc., so all potential causes for unresponsiveness still must be considered, even if we know the patient is diabetic.