Halkenberg using his ability on himself is quite risky imo.
I'm guessing they found out what happens when a body switched Halk guard kills that hostage body. They were waiting to see what would happen to the guard's body after the first hostage body shot himself infront of Luzurus's room.
Halk switching himself is dangerous because,
Either his Nen Beast goes with his soul, and so it also switches to Balsamilco's body. In that case, even though Benjamin wouldn't be able to see the nen beast, all of Benjamin's guards will be able to see the nen beast above Bal's head, and that will basically at least alert them to the fact that it is not Bal, if not out right let them know that it's Halk.
Or, his Nen beast stays with his own body, meaning the now comatose Halk body with presumably Balsamilco trapped inside it.
And Wario did say thay Halkenberg was gonna die. And FakeBal(Halk) also promised Benjamin that Halkenberg would die soon.
So Halk's guards are planning to kill his original body with Bal inside it. If Halk's nen beast is connected to his body rather than his soul, then the nen beast is gonna die with the og body. Which will leave Halk insanely vulnerable, as well as unable to use his ability any longer.
I believe that nen beasts are tied to the soul rather than the body.
In that case, there is another possibility. That Halk's nen beast hides it self completely from Banjamin's guards.
Tubeppa's beast has shown the ability to hide itself. And Woble's beast hasn't even appeared yet apart from that small instance where Kurapika felt an ominous aura near Woble for an instance.
So it's definitely possible. Halk could also go into Zetsu for a long period, thus cutting off the aura to the beast. But i don't think he would be able to achieve that long of a zetsu. It took Tserriednich some time to get to 10 seconds of zetsu. Even if Halk is the secret child of Beyond, i doubt he could master the basic principles so fast that he eclipses Tserriednich's growth rate. Maybe.
And i assume whatever Halk in Bal's body has planned, would require at least an hour or two if not way longer. The obvious action for him is to get close to Benjamin and kill him. Even if Halk in Bal gets Benjamin alone, Benny is insanely strong physically, has nen and Benjamin Baton that gives him access to multiple nen abilities, and has a monster looking nen beast too. It will be incredibly hard for Halk in Bal to kill Benny. That leads me to think that maybe he has another plan. Maybe utilizing the head of royal military master sergeant/Benjamin guard's captain position or something.
And if they do kill Halk's og body, i would guess that if Halk in Bal were to die as well, he'd just straight up die. In stead of possibly returning back to his og body. Or maybe they can kill the og body, show to Benjamin that it is indeed dead, then somehow revive the og body/keep it alive. And after Halk in Bal finishes whatever he intends to do and then kills himself, or gets caught and kills himself, he can go back to his og body and be Halkenberg again. If it's that way, that's an incredibly broken ability that Halkenberg has...
idk