Would you keep the scene where Leia speaks to her envoy about getting help from the Republic?
I feel like the dialogue would still work. Now the Republic thinks she is insane for wanting them to take action with this metaphorical gun to their head.
If they did know about Starkiller and the threat of the First Order, it might raise the question as to why they still had both their Senate and their fleet in one location, or why they did not build up their military defenses in response. At least the Republic thinking they were safe explains why they lacked many defenses. To be fair those questions can probably explain away similarly to how questions for the theatrical film are explained away.
Also, I wonder if Starkiller should get so much focus in the crawl even though it doesn’t become important until the second half of the movie. (I could see how a hypothetical first-time audience could immediately be turned off by the movie drawing parallels with ANH from the very start.) Though once that plot gets rolling it will fit nicely with what we do see. The first thing Leia asks Finn about is the weapon, clearly being a major priority for her. Hux’s speech would also fit with this idea, with Hux using the Republic’s tie to the Resistance as justification for their attack, as if it were a break in their armistice.
If Starkiller’s existence is common knowledge, it might could help explain how the Resistance comes up with a plan to destroy it so quickly. This is something they’ve already been researching, and they just needed Finn’s insight to hammer down the details. In the theatrical, although the Resistance might have already speculated on the weapon’s existence, it is not made very clear in the film itself that they knew about it until after it blew up Hosnian Prime. Although, what would be the explanation for why the First Order chooses to hold the Republic hostage rather than destroy them as soon as it was complete?
I do think this is a good angle though that could be explored further! I don’t think having the Republic being held hostage by Starkiller is an approach we have really considered before. You’ve been having a lot of good ideas lately so I would say explore this path. If we gotta have Starkiller, maybe the best approach is to lean into it more!