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This launch seemed to use a much steeper trajectory. Typically they go downrange much faster than they gain altitude, but today they seemed to go up faster than they were going downrange.

How does this affect the fuel efficiency of the launch? Is this a way to cut down the horizontal velocity so they first stage can return more easily?

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It's a product of a higher target altitude (650km for Orbcomm vs. 330km for an ISS resupply) and wanting to perform a single second-stage burn to achieve the target orbit.

Maybe. This time, the satellite load was just 1.1 tonnes (vs. 6 tonnes capacity), so they probably had more room for maneuvering and hence decided to do stage separation closer to the launch site in order to aid with the recovery attempt. Or maybe it's standard procedure on lighter payloads; perhaps it reduces maximum dynamic pressure.

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