What ending did you choose? What is your take on it?
1. There are setbacks but overall the ending isn't really dependent on getting the panacea early. Remember that its ultimately Bachelor who "diagnozes" the illness of the town by identifying that the Polyhedron can be destroyed to reveal a wellspring of blood. Getting the panacea early saves Rubin and gives you 2 additional panaceas to go around come the final days.
2. The main gameplay is more or less the same in 1, but due to much fewer choices and the lack of certain features like sprinting and boat travel its a much more tedious and less engaging experience. On the other hand 1 is also much easier to break and have a risk-free time with, both because it allows to savescum and because you can farm resources pretty easily once you know how.
3. Never played the original Haruspex campaign but overall quite a lot of plot points are different even if they end up leading you to the same thing. The townspeople turn on Haruspex quite a few more times in the original, for example, at some point even imprisoning him (not just on day 1). The Minotaur/Foreman is more of an antagonist at the end who has you injected with various poisons and has you run trials to have you prove yourself as his replacement and then you have an edgy boss battle with him (pretty gamey shit, 2 does it much more elegantly). There is also a memorable plotline where something really tragic happens due to a complete coincidence - actually, a few times - actually the entire fucking game is a string of these events - so while a lot of people miss its inclusion I think 2 has more than enough of its own new tragedies to replace that with (getting Lara a supply of water in particular is pretty OH FUCK)
Like I alluded to previously, the biggest change by far is the main question posed by the game. The endings of both games are the same (destroy the Polyhedron or destroy the Town) but the focus and implications are different. 1 was all about a utopia. What does it take to create a perfect order on earth? How far would you be willing to go to make the world you want to see? For post-soviet people this is indeed a very compelling question. In 1, the focus is either on preserving this great and terrible new order of the Cains - regardless of the fact that it either caused or aggravated the Pest by the sheer impossibility of it - at the cost of the old world, or you maintain the old, stagnant status quo by sacrificing the Polyhedron. In 2, while some of that dilemma is present, the focus is much more on transformation, fantastical vs reality, and a kind of autopsy of man. The town is a machine - humanity used to labor within a kind of primordial environment until they decided to become their own environment and created towns, which are systems unto themselves (this is reinforced by the many hierarchies, traditions, and systems of oppression created solely to keep the town running - people created the town but now the town is shaping the people). Is this humanity's yoke or is this system just as deserving of perpetuation as the state of nature?
The interesting bit is that 2 completely turns the progress vs order dilemma on its head - preserving the polyhedron now is simultaneously the progressive (preserve the impossible miracle of the cains) AND the regressive (destroy civilization and revert humanity to a baser form) choice...at least according to Haruspex' perspective. Note that I am being purposefully vague about the true value of the Polyhedron - which is elucidated on more in the Bachelor's playthrough - and of course much of your view of the game varies based on your own personal interpretation.
4. Every quest/dialogue is different although many general events intersect and you get to see them from a different perspective. For example as Bachelor you will interact with the Haruspex, at times aiding him and at times working against him and at times accidentally fucking up his plans much like as Haruspex you did the same to Bachelor. The ending is the same. This is important because instead of giving the characters different endings, what they get instead is different sets of facts. As Haruspex, you are supposed to find the destruction of the polyhedron and the preservation of the town a much more persuasive choice. As Bachelor, for reasons I won't spoil, the opposite is true - this is why the Bachelor begs Haruspex to preserve the tower on the final day. You will get a very different perspective on the whole dilemma and its going to be, like, really sick, yo. This is by far one of the most interesting parts of the game for me, the way it plays with perspective to convince you of a certain character's canon ending as opposed to just forcing you to pick something different from last time.
5. In 1 the cast was the same. However, the characters each protagonist primarily interacts with are different. Bachelor had way more to do with the elite of the town and the Cains in particular. He also saw a much less sympathetic view of the Vlads (old Vlad in particular) and of the Steppe people. In the original the Bachelor never visited the Abbatoir or spoke with the Bull-Foreman (I don't see why that would change in 2). What he did do was visit the Polyhedron, where Haruspex never goes. Meanwhile Haruspex interacts way more with the children and core town's people, goes to the Abbatoir, barely talks to the Cains, etc etc etc
6. Its complicated. In 1 you were locked into your "canon" ending (Bachelor: destroy town. Haruspex: destroy the Polyhedron. Clara: [REDACTED]) as long as you saved the people on your list (if you remember, for Haruspex its the kids of the town). However, if you play efficiently and also save the people from another character's list, you get to pick their ending, too (except for Clara's because [REDACTED]) There are also a few extra narrative secrets that get unlocked for each group of characters you save which I won't spoil.
In 2, once again things are much more elegant. Haruspex can always choose between the two endings available to him, but they are affected by the survival of each character. In essence you end up with your own personalized ending variation based on each character you save. Save young vlad and he will implement a bunch of reforms to make the city's industry less exploitative. Save the kids - as your dad wanted you to - and the town ends up with a future. Etc, etc. In my case, it just so happened that nearly all of the old generation died (the saburovs, Old Vlad, georgiy Kain, etc) and so did the steppe characters, but their children survived, which I thought was a fucking perfect representation of rebirth - just like dad wanted, the plague inoculated the town of its traditional roots and forced it to grow new ones.
