Yes, you can use the local world to show that map info, if a player is close enough to see the map, he should be close enough to have the chunks sent to him right?
Use the client side chunk data to determine simple terrain stuff like that.
No, registerEntity does not replace registerEntityID()
It register the entity for tracking, and that information, and makes it so that you can not care and just use a random number for the ID in registerEntityID.
The ID you give to MinecraftForge.registerEntity is a mod unique id instead of a globally unique ID. So you can use 1, 2, 3 etc..
But in registerEntityID you can use Random.nextInt(35565) for all MC cares.
As for your other questions, you can check EntityTracker on the server to see some example values for normal mobs.
As for the velocity info.. is your mob velocity sensitive? Like Minecarts, or boats? If not you prolly don't need that info.