Who's winning the war?

Assuming you have finished chapter 3 (because the stories seem to take place at slighly different times).

Then the Republic is winning

Yes because World War 2 was fought for thousands of years.

Semantics

While I acknowledge that the story may very well do this, because it is, after all, BW's writers' prerogative, I would like to point out that it's pretty silly for a single technological bump to be the difference between imminent defeat and turning the tide, and that such a thing has zero precedent in actual human military history for good reason.

Semantics

It didn't work out simply because it came too late and there was no time. If there would be enough time and resources (factories not mostly destroyed by Allies), just those new fighters could've made a lot of difference since we know that war in the sky was one of the biggest advantages for Allies by that time.

And this is not just fighters that will be influenced by isotope-5

Really now? Because I seem to recall Japan surrendering after the United States created (and used) the atomic bomb. The Germans ran over the polish in WW2 because they had tanks while poland was still using calvary. And I'm pretty sure that if the Empire had the death star right now the republic wouldn't stand a chance. For you to say technology can not turn the tide of a war is just downright stupid.

That is precisely my point. Germany possessed superior technology in some ways, but its industrial inferiority and Allied military advantages in other areas meant that it could never be leveraged in a meaningful way. This suggests that it is not merely enough for a state to possess good technology; it must do something (and must be able to do something) to use it properly, and that technology must not be negated by enemy advantages in other areas.

Much like Nazi Germany, the Empire's military is reeling from serious defeats, Imperial forces have been pushed out of the major industrial areas of the galaxy, and Imperial worlds are about to be subjected to Republic attack. Hitler's Wunderwaffen did little more than kill a bunch of Londoners indiscriminately; even if we suggest that the Empire is much more capable of leveraging isotope-5 technology than Nazi Germany was of leveraging rocketry, jet engines, and superheavy armored vehicles, that still leaves a hell of a gap between "maybe managing to stall the Republic offensive in some places" and "completely turning the tide of the war".

Japan was doomed long before the American nuclear attacks on Hiroshima and Nagasaki. The Imperial Japanese Army was badly overstretched and hemorrhaging its strength uselessly in China, Burma, and on the Pacific islands. The US Marines and Army had driven the IJA's garrisons out of countless Pacific islands and seized control of launching points within ancestral Japanese territory itself on Okinawa and Iwo Jima. The Imperial Japanese Navy was gutted in 1943 and virtually annihilated in 1944 by overwhelming American naval power and superior doctrine. American air forces were in the process of constantly pounding Japanese cities to rubble by conventional arms. And, of course, the Red Army launched its own attack on Japan at the same time as the American bombings, and shattered what was left of the Kwantung Army in Manchuria. Japan's military strength was destroyed because it tried to fight literally every single one of its neighbors, any one of which possessed numerical strength that could dwarf anything the Japanese could bring to bear. Imperial Japan's fate was decided when it went to war with the United States and the Commonwealth in December 1941; the only thing that was left to be determined was how many more people would die before the Japanese military was finally either destroyed or compelled to surrender. That was what the atomic bombs decided. They certainly did not "turn the tide" of a war that Japan might very well have won otherwise; Japan was a broken country in the summer of 1945.

For a demonstration of why Japan's military effort in the Second World War was, ultimately, not ever in a serious position to "win" any of the wars that the Japanese military enmeshed itself, even the China war, I would recommend two books: Soldiers of the Sun, by Meirion and Susie Harries, and Japan's Imperial Army, by Edward Drea. The former book has aged somewhat, but it is still reasonably accurate on many points. On the China war itself, the collection of articles in Peattie, Drea, and van de Ven (eds.), The Battle for China, is fantastic.

The "Nazi panzers against Polish cavalry" thing is a myth, pure and simple, invented by newspapermen and magazines of the time to romanticize the quixotic Polish war effort and later seized on by Nazi propaganda to demonstrate how futile Poland's fight was. It did not ever demonstrably happen. Poland's army, while it was not as modern as the Wehrmacht, possessed many light tanks and antitank weapons. The Germans actually had more cavalry units than did the Poles, and German units relied on horse transport to a remarkable extent even in the later stages of the war. Simply put, the Poles were outnumbered and outmaneuvered from a very bad strategic position at the start, and fell victim to superior German doctrine.

I would strongly advise that you read the posts you respond to more carefully, and that you have a better grounding in the subjects that you deride others as being "stupid" about.

That is precisely my point. Germany possessed superior technology in some ways, but its industrial inferiority and Allied military advantages in other areas meant that it could never be leveraged in a meaningful way. This suggests that it is not merely enough for a state to possess good technology; it must do something (and must be able to do something) to use it properly, and that technology must not be negated by enemy advantages in other areas.