If you only need the mouse broadcasting, I'd see if you can find a script for HKN that will do mouse only.
That's all you need, if you're otherwise happy with what you have.

IS Boxer is amazing, and its relatively easy to configure to do what you want.
I would say, HKN and ISB are roughly equal; have not found anyone post that theirs can do something the other cannot.

But IS Boxer is subscription based, HKN is free.

You can apply for the 7 day trial of IS Boxer to see if its for you.
I wouldn't go this route purely for the mouse.

But IS Boxer offers a lot more then just the mouse broadcasting.
Keymaps can essentially do whatever you want them to, without needing to devise your own scripts.
PiP swaps are instant between clients.
It is a very nice boxing program.
I have a lot of Keymap examples, and a wizard walk through in my guide.