Some members of the South Korean Government are controlling the Omnics that are attacking South Korea and it’s neighbors and are in league with Talon.
Talon is trying to start a war between Omnics and humans. Having an omnic attack a country or two over and over gets Omnics into the news in a negative fashion.
If you create an outside threat, people will unite around you which can allow you to hold power for longer.
Additionally, by controlling the Omnics that are attacking you can use them to eliminate rivals/threats to your power.
This would also possibly explain the force selections by the South Koreans. People could have become bored/disinterested in hearing about the countries drones fighting against the Omnics and winning. Getting a bunch of celebrities to do the fighting on stream would get people to care about it again. By increasing the challenge of fighting the mechs you’d get increased interest. Having it take all but one of your mechs taking serious damage to get the win would put on quite the theater.
If you were actually being attacked on a regular basis from the sea, it’d be more likely you’d employ land based defenses to beat it if it gets within attacking range or long ranged aerial defenses to prevent it from ever getting close. As this is in the future, a rail gun based defensive installation would be entirely possible.
Hana likely wasn’t supposed to be at the facility when the attack came. The Omnics that were attacking managed to basically disarm her ship but, didn’t actually hurt her which could have been intentional.
The attacking force was also tiny. Unless you knew that you weren’t going to meet significant resistance that was a pretty much guaranteed fail from the start. You shouldn’t use new tech on an operation that is likely to fail unless you have limited other choices.
Unless the attacking Omnic is destroyed or something significantly changed it’d be unlikely for Hana to leave Korea which would likely be necessary for her to join Overwatch. If she finds out that the government is running the Omnic and is branded an imposter that would work at getting her out of country and she’d fit in with Overwatch being considered a rogue organization.