If you're going to allow input broadcasting to stay in the game, at the very least balance the game around the fact that it will exist.
One example I will give you:
Garuk's Improved Viral Assault
75% NR Check
Guaranteed cap on most targets
Aimed shot is the other obvious candidate
In a game where input broadcasting is legal, things like this should not exist. Yes it will hurt the NTs who are playing with one account, but that's the consequence you have to deal with for allowing this.
When you create uncounterable play, it's taken advantage because people like to win. When this uncounterable play requires an unreasonable time and money investment, you will see the people who don't choose to multibox leave the game.
Removal of assist in PvP was a small shuffle in the right direction, however Multiboxers will still have an extreme advantage as all their characters on follow will be tab targeting the same target. That's not something separate players can emulate as we are all from different locations and 99% of the time not on follow.
I left the game at the dawn of multiboxing, most of the bigger clan pvp orgs either quit or stopped bothering with NW and I come back 3 years later to see that real players have just been replaced with people willing to fund multiple accounts because they've been given the green light.
FC is going bust anyway, at least for the few months this game will stay alive: make it fun again to pvp (for everyone).