How to hurt Zann's power base
1) Bounty Hunters and assassins. G0-T0 could easily send HK's to investigate, and from there sabotage and assassinate any who stand in G0-T0's way. This is the only real way I see it, but in all honesty, I don't see the point. Maybe a few, but not an invasion force. If G0-T0 stays on Nar Shaddaa, Zann will be forced to attack, since G0-T0 won't leave. This gets rid of Nightsister/rancors(I did this anyway), tanks, and a majority of Zann's army.
Your scenario (let's pretend) is flawed as well. So Hanharr sides with Zann (even though he won't) so what? HK's are droids. Droids programmed to kill. If something gets in the way of that kill, they are killed. No question. So, shoot Hanharr, then Urai. Send multiple HK's, and have them hide in wait. Hanharr begins doing whatever you say. HK's notice that Hanharr isn't fighting. That makes them shoot both targets. If Urai escapes, he ain't finding the HK's. Even with force sensitivity, they are droids, so no sensing.
But if G0-T0 is fighting a gang war on Nar Shaddaa how can he also be messing up Tybers power base? Tyber is coming from a much better starting position the only way G0-T0 can win is if he fights a guerrilla war. Which Tyber can defeat by bribing his forces and hunting down G0-T0's strong hold, which he then attacks in force (why would Tyber NOT use Nightsisters on Nar Shaddaa? Rancors on Skyscrapers sounds like win to me) and forces G0-T0 to defend him self or lose his only power base.
Another way is to find and blow up the HK factory, we know from the in-game content that the HKs have a homing beacon that can be used to trace them back to their place of origin, so Tybers' techs figure it out and then BOOM, no more HKs.
Also jedi can sense DANGER as well as intent. Ok well 1. I had too scenarios; The first had Hanharr side with Tyber before he even gets to the planet, in which case he could just destroy/sabotage the droids on the way there. In the Second one Urai didn't face Hanharr until AFTER all the HKs were destroyed, so either way the point about them is moot.
Also why would Hanharr not join Tyber? He can offer him just as much (if not more) bloodshed then G0-T0, who keep in mind likes order not chaos, so Hanharr would be disruptive to that. Maybe Tyber could bring that up "Hanharr, you are a mighty warrior, but surely you must realise G0-T0 will kill you after this join me and you can kill to your hearts content." OR some such argument.
G0-T0 doesn't need to leave Nar Shaddaa. But his forces (aka HKs, Ubese bounty hunters, Gand, Exchange, Zhug Brothers) will move across the galaxy to hurt Zanns powerbase. Also remember that in order to win G0-T0 just has to kill Zann. G0-T0 is all about endgame.
There's no winning by default. Traya stayed on Malachor and she won. Somehow. Anyways, staying in one place does not make you lose by default. In this case, it'll make G0-T0 win.
What game were -you- playing? You don't bribe Hanharr, he comes with you because he wants bloodshed. He doesn't care about credits, just battle. And siding with G0-T0 will get him many more people to kill than siding with Zann. There's no reason for him to defect.
His 'forces' can be beaten and Nar Shaddaa placed under blockade, as the planet starves (major issue for a city world) what will G0-T0 do?
Also the whole Traya thing bugs me; If you have contained the enemy threat why not wait them out? I could buy Revan being rash or impatient or just think he was unbeatable, but Tyber he just say 'Let him rot, I've got a galaxy to corrupt. HAHAHAHAHA" Also Revan surrendered, even though it was explicitly stated in the op that you can't/won't.
Any way, I never said you bribed Hanharr, he was working for Voga the Hutt the entire time. My point was that he was not loyal to G0-T0 the same way Urai is to Tyber. he wants to kill and kill a lot, G0-T0 likes order Hanharr is not conducive to order, thus he will kill him after the war. Also Tyber can offer him just as much if not more bloodshde/chance to fight Urai then G0-T0.
Also about influence, G0-T0 can influence markets and economies but he can't effect regime change directly. He could work for months to position his pieces and set up for a friendly government to take over a planet, then have Tyber come in and take out the new guy with his army.
A fun way I could see this ending is Tyber and G0-T0 teaming up to take over the Galaxy. Tyber will provide stability and military forces will G0-T0 manages the books. They would be a joint entry, meaning that they wouldn't have any allies as one fills that spot for the other.
"Jedi, Sith, Empire, Republic. Their time has past now comes the age of the Consortium." The full name is the Zann-G0-T0 or G0-T0-Zann Consortium.