Minecraft – Can i get the horse into a boat

You can boost a thrown Ender Pearl using a TNT cannon. Assuming you've got the trajectory exactly right and have a wall at the destination to stop the pearl, this is basically instantaneous other than the fuse time (4 seconds). Also, in one step you can only travel as far as the chunk load distance, which as far as I've read is 160m (10 chunks), or else the pearl will not land until you move closer. So, speed: 40 m/s average, less time to gather resources and place the TNT, and the flight time of the pearl (which will depend on the amount of TNT used and exact trajectory).

If you built a series of redstone-driven TNT magazines wired to operate in sequence and launched pearls perfectly to match them, then you could avoid waiting 4 seconds and move even faster.

This is due to the South-East rule. Despite a lack of mention in that page, this quirk does affect redstone. What's happening is that the dropper on top receives an item from the bottom dropper before it's instructed to fire when the comparator is on the north or the west sides of the droppers (opposite to what you said). When the comparators are on the east or south sides, the redstone power is cut before the top dropper is instructed to fire for the last item.

But it's more complicated than that. It's not just where the comparator is placed in relation to the dropper, but also where the rest of the redstone is placed. I've set up the same elevator, but mirrored, and it no longer emptied the top dropper.

My suggestion would be to either use a different item elevator, or find which of the 8 versions is going to work best for you.