I think the current use of start/end attributes of the <animation_clip> is not the best choice. Animation are referenced and this means they can come with another file, e.g. in an extern model file with animation templates for the model. The current method requires an absolute time over all COLLADA files in the collection.
I think to assume every animation to start at time 0.0s, and to set a time offset with the start attribute suites for more situations.
See discussion here https://collada.org/public_forum/viewtopic.php?t=941
As I said in bug 57, The design of animation to-date does not include the next steps of defining a system of animation blending, media time-line, etc.. as you are noticing.
I replied to your post on collada.org asking "You can add per-instance <extra> data to the <instance_animation> to transform the animations in time. Have you tried that?"
The work group is deciding a schedule for specification updates of 1.4 and 1.5.