Regarding your first concern, the param disableTextAnnotations is working as intended since you are receiving in your response just the textAnnotations and fullTextAnnotation. This parameter is intended to disable the old textAnnotations field but the full_text_annotations will be returned [1].
Regarding the 2 and 3 questions, Google Cloud Vision client library does not have an option for requesting partial responses like the ones you are asking. Nevertheless, I would like to suggest checking [2][3]. Those are similar requests that could help you in order to find a workaround.
Hope this helps!
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[3] - https://stackoverflow.com/questions/53056817/limiting-the-response-from-google-cloud-vision-api