Just to be sure I'm not missing something - it doesn't seem possible to adjust temporal easing with keyframes in Motion templates. There is a dropdown that let's you select "Straight" or "Smooth". But as far as I can tell it doesn't do much, making me think it only applies to Spatial Interpolation, not Temporal.
Temporal Interpolation always seems to Ease In and Out. Which is what I want most of the time, but not all the time. At this point I'm dragging my keyframes outside the clip, so it's full speed when the clip starts and ends.
Were you able to get an answer to this? I'm not confident I understand the question completely, but the Keyframe editor is where I go for those adjustments. This article explains it: https://support.apple.com/kb/PH16083