As far as I am aware the retiming behaviors and the timing in the properties inspector are applicable only to clips not objects. If you have a background clip you could use a tracking behavior on your object(s) and track an object in the clip and retime the clip. To do this you would have to duplicate the clip and mask the object being tracked. The easiest way is to set your keyframes for 30 sec. Basic motion speed is controlled by the keyframe placement. Some of the behaviors have a speed control, so If you can find a behavior that accomplishes the motion you are trying to achieve that could be a solution as well.
The length of time it takes an object to travel a motion path can be controlled in the "mini-timeline" under the canvas. Placing the cursor near the end will change it to a "trim" cursor and you can click and drag the end out for as long as you need Motion to "play" the effect. [*select the behavior in the layers list first...]
Well, I know you can stretch out the motion path behavior to do this but I didn't enable a motion path. I just turned on live keyframing and moved the objects around in 20 different places. When I stretch the object timeline, it just adds more time for me to add more keynotes. What I have is 3 layers with keynotes on the x & y position and I want to be able to grab all 3 of those layers and stretch the keynotes so they have the same relative speed but the motions just happen a lot slower. Maybe I should have started with a motion path behavior :/
Since you created your motion paths with keyframes and you want to extend the motion from 15 sec to 30 sec, then use the keyframe editor (click the diamond icon button at the bottom right of your window) Then select all the keyframes (shift click each one or draw an outline around them) and using command-arrow keys move the keyframes out so the end ones are at 30 sec.
Ok - no problem.
Below is a simple shape with a simple "Motion Path" converted to keyframes (white arrow).
Clicking the icon under the blue arrow will reveal the keyframe editor.
With the keyframed object selected in the layers list, click the icon under the green arrow [Transform Keyframes Tool]. (The colored lines under the yellow arrow show the original keyframe set before editing.)
Click and drag a selection around ALL the visible keyframes. A white framed rectangle with drag control points at the corners and midpoints will appear (see under orange arrow.) Click and drag the MIDPOINT on the right side and drag out to the time you you need. All the keyframes will be adjusted "linearly" and evenly throughout the range of the time expansion or compression.
Note: You can use the Transform Keyframes Tool to control selected keyframe points without affecting unselected points. You can also click inside the Transform Keyframes Tool and drag the selected points to new locations, thus keeping portions of an animation intact with respect to each other.