Leaderboard

The client takes input, converts that to motion, and sends position updates to the server. Yes.

This is due to the fact that the client does not inform the server about it's position change every tick, but rather once every couple of ticks.

What do you mean by not taking skpped ticks into account? Are you talking about when there is lag? People argue about it but most end up agreeing that if the whole game is slowed down then you should scale accordingly -- if the ticks are only 10 per second then the player speed should be half as much. If you really wanted to calibrate to real time that is fairly easy to do though. Minecraft is just a Java program and Java has various methods for checking real-time. So you can create a timer that checks for difference in system time on each server tick and if it varies from the expected 1/20th of a second you can set a scale factor you can use for calculations during that tick.

Isn't there a motion field in Entity(Living)?

You can set the acceptedMinecraftVersions property of your @Mod annotation to change which versions of Minecraft are allowed to run your mod. If your mod isn't actually compatible with a Minecraft version, it will likely crash at runtime when it encounters an incompatibility. Look at VersionRange.createFromVersionSpec for brief documentation of the version range format. Look at FMLModContainer#bindMetadata to see how FML replaces some specific version ranges with others (where multiple versions are compatible with each other).