You mention you're limited to short videos due to issues with the maximum size of images on ios. Why not just generate multiple images? That way you could load the video "parts" in parallel as well.
On iOS Safari you're limited to 2 parallel HTTP connections at a time so not a huge advantage to the multiple connections. Don't disagree with splitting it up though
Yep! If you have a long video, its better to split the sprite into multiple parts.