Park Chung Hee was the 4th [including an acting] president of South Korea
.....
[For me, yes if they knew his service with the Japanese and Manchukuo Armies.]
Allow me to first introduce to you what happened OTL in the post-WWII Korea...
The Provisional Government had been, by this time, biding for several months to plan an "attack" where they would go deep behind front lines and instigate a rebellion within Korea. When they heard Japan had surrendered, the Koreans knew this was all over.
Three things happened during this time. First, the Koreans were caught between various power blocs and ideologies, fracturing the political arena and bringing forth what we now know as the Korean War. Second, both the Soviets and Americans did not and were unwilling to understand the Koreans; no serious study was done by the Americans, for one, on Korea or its people. Third, situations forced it so that sanitary and food situations on both sides of the 38th parallel were heinous; the Allied forces had barely prepared or the welfare of the people they were to govern.
So what happens? Lyuh, in his rush on making an agreement, forgets to put a clause stating when a Korean government would replace the Japanese one. So a loophole was made and abused by the Japanese- they argued that they will surrender control to the "Allied forces", namely the USSR and USA. They were not going to surrender to Koreans. Different factions began to emerge when they did arrive; between those left and right, between those who left and stayed behind, between moderates and radicals; Korea as a political arena was absolutely catastrophic south of the border, and everyone was assassinating everyone else.
What else happens? The US and Soviet Union were so unprepared and ignorant that they viewed Korea as another Japan. You probably heard of how two minor soldiers were the prime reason for how the 38th parallel was chosen to be the dividing line between Soviets and Americans; well, other things they did were just as bad. Thinking Korea as a Philippines and ignoring(or unaware of) the continuous independence movement that was going on, the US thought Korea should have a trusteeship for at least 30 years.
And this happened. Because of the simply atrocious quality the Koreans were thrown into post-war(the US simply being prepared to "liberate", not save the population, along with prevalent corruption), there were many major riots across the nation. Especially, the
Autumn Rebellion of 1946 saw nationwide unrest. With the Americans only being intent on securing their military in South Korea, and with southern Korea being the historical base for Socialists and Communists, anti-left massacres and other atrocities were inevitable.
The local independence leaders were simply ignored and both sides of the Allied forces brought individuals whom they thought they could trust. Kim Il-sung and Rhee Syng-man...what interesting choices. What bad choices too, since both of them were not very loyal to their superiors. They were, as a matter of fact, very rogue leaders- Rhee Syng-man, also chosen as first President of the Provisional Government, was ousted due to abuse of his powers and fracturing the weak alliance different political groups had. Kim Il-sung would also prove same, when he purged any major political players, even those that were close to him, by the early 60s.
For our convenience, let's look at the situation of Rhee. When he arrived with the Americans on Kimpo Airport, there were only two things going for him: the full support of the American government, and his ultranationalistic views. But there were so many things that were against him- a strong domestic political arena, the lack of support from anyone within Korea. So he went for the most base, most vile option: he, with support and encouragement from the United States, brought many Japanese collaborators back to power. Since he didn't have a power base, and all others were taken, he was forced to make one.
In this situation came Park Chung-hee. Once a Kwantung Army officer and now just a Japanese collaborator, he used this chance to rise back to glory in the Army. Leaving more details aside, Rhee's choice to use Japanese collaborators instead of purging them, along with full American support behind this decision, brought "convicts" like Park back into the political and military scene.
How to stop Park? Well, what use is there? Every other option other than Park as president- Paik Seon-yop, etc. were just as aggressive, anti-democratic and driven as Park ever was. They were men who saw the glory of Japan, its quick industrialisation and sole aggression as an Asian nation against the West-dominated world, and the rise of Communism. And they were soldiers. There is no chance in the world, with any POD, that Park or people like Park were not going to rise to power. Everyone knew what was going on- Japanese collaborators rising back to power- but what can they do? The Americans were supporting it, and they needed the US support against the Communists.
And if we were, for instance, to have someone like Jang Myon become president- the military, the strongest institution in South Korea, would be against it. Filled with ex-Japanese soldiers, they were virulently anti-Communist and did not see moderates as fit to rule a country; it would be coup after coup, similar to what happened in Thailand.
And is there any use in that? I think not.