1 pointYes, when you're registering entities one of the things you specify is the entity tracker, which includes the range. But as diesieben07 pointed out, you can't get the rest of the players unless the server sends it (which requires a server-side mod) so your original original idea is only possible if the radius that you're interested in is less than the tracking distance for the client. If that is true, then you're good to go.
1 pointIt depends on the tracking range of the entity when the server starts sending it to clients. For players this is 512 blocks. This command needs a server-side mod to work properly.
1 pointAlso, check the output of java -version from the commandline to make sure you have the 64 bit version installed (even if you have 64 bit windows, it is possible to have the 32 bit java). If the output of that command does not say 64-bit anywhere, then you are running the 32 bit and need to get the 64.