Most TV and movie scifi with time travel admits changes in the timeline, and delight in the paradoxes. It's rare to find one rigorous enough to limit itself to a causal loop, which may be wild but isn't a paradox. Voyager is quite typical in the way it admits some paradoxes while excluding others, seemingly unaware of its inconsistency. I'm not sure why Endgame, one of many time paradox stories, is somehow so offensive. Until that is explained, I'll just move on.
Nor am I quite sure why it is so necessary that the Borg be unstoppable. Adaptability
was always emphasized in the presentation of the Borg, and the Borg adapted quite rapidly in this episode. They were never presented in Voyager as invincible. In any event, the Voyager episode that showed them at their weakest was Survival Instinct, which is never cited as an example of the error of making the Borg conveniently weak. Since it is evident this is not a serious objection, I'll just move on.
The Admiral's attempt to change the past her way was rejected, and she dies, so it is unclear how her position on changing timelines was endorsed. It is especially unclear why the episode was demonstrate in some fashion that she was right.
It is clearly stated that the spacetime travel was to be one way, and there would be no return. Therefore, clearly the Admiral had to travel to a point in their journey where there was a way to come back. The Admiral was in no position to agree to Q on either occasion. The decayed wormhole still would have been decayed. And diverting Voyager away from saving that planet from the Ferengi would have been loathesome.
Quite aside from the fact that somehow interfering with the original capture of the Voyager and the Maquis would have meant sacrificing both
Kes and Seven of Nine, and Icheb too, what could the Admiral have done? The Caretaker reached out over seventy lightyears to grab up Torres and Kim for reproductive purposes. She couldn't have gone back to the Alpha quadrant and warned the Maquis, who were after all hiding as well as they could. It's not even certain whether the displacement wave wouldn't have simply struck at Earth if Voyager had sacrificed the Maquis!
As to interfering at the Array, there's no way she could count on overcoming the Caretaker. After the Caretaker's death, events were forced by the Kazon. How could the Admiral know that her shuttle would be enough to destroy all the Kazon reinforcements, even if should she want to resort to such violent methods?
(By the way, the acquisition of Seven of Nine was probably the key event in Voyager acquiring enough tech to defeat the Borg on its original trek. Terminating the journey without Seven of Nine would begin a timeline in which the only source of that tech would be a future that didn't happen, a full blown paradox. As is, the new timeline had a wealth of data to use against the Borg, including the Infinite Regress and Collective/Child's Play weapons. A possibility, for what it's worth.)
The Admiral made it clear that she needed to push the agenda to keep the Captain off balance. That alone explains why she didn't go back early enough to save Carey. And the episode showed us that keeping the Borg off balance was at issue as well. I don't understand why people ask the question. Not liking the answer is another thing.
Two last issues: C/7, and the resolution. C/7 was present since Human Error, which I thought was far more offensive. It played a much smaller part in Endgame and can't be such a big deal. Insofar as Voyager had a continuing story, it was about getting home. The series ended at the resolution, the climax of the story. The rest was basically showed to us and will not be essentially different: Paris and Torres have a baby who grows up to go into Starfleet, the Doctor practices medicine and sex, Barclay doesn't stutter, Tuvok has a dignified old age, Janeway becomes an admiral, Kim becomes a captain, etc. I really cannot believe that people were disappointed at not seeing Kim with his family, or Seven with hers. After all, we had seen those meetings in previous episodes!