If you get a job offer, I think ur intl office should be able to expedite the OPT application process and you will get ead card within couple weeks, regardless 90 days is little far fetched and you should get the card within 1-2 months if you apply without a job offer letter. As far as when to start, if you don't have a job yet it's better to start from Oct 1, just to avoid the gap next year when your OPT expires and H1 starts, if you don't have any h1 aspirations however and are going for grad school or sth then you should startOPT sooner.