Take _Star Wars_ for example. You don't need to know how the Force works. There's no suspension of disbelief because you've bought into the genre and its tropes. The Force is everywhere, Luke is atuned to it. We don't need to know anything else mechanixally. Then they rolled up with midichlorians and reduced the whole thing to jibberish.
_Interstellar_ sets a hard sci-fi expectation with a visually accurate black hole before losing the train of thought. Imagine if _Lord of the Rings_ spent 20 minutes discussing how the plate tectonics of Middle Earth gave rise to Mount Doom before veering off into the history of the eagles. It gives your audience whiplash.
The small craft could launch from a planet with twice the Earth’s surface gravity, make it to escape velocity, and meet the Endurance?
Why would they need better propulsion than that to leave the planet and build space habitats?
Under these circumstances (only a couple years left until starvation begins & no existing industry) it seems absolutely realistic that the government is able to throw together one hailmary in secret, without also being able to scale production up to save most people before they starve. Remember, the goal of the mission wasn't to measure the data they need to finish their equations, it was to send out one single ship that could continue human civilization on an inhabitable planet.
Had the future humans not intervened (by placing the black hole near inhabitable planets), humanity on earth would have died.
Not only a black hole, a supermassive Kerr black hole with relativistic rotation at its equator.
They could have done a better job at the planets themselves though - Most of them were pretty useless for terraforming.
While I'm sure the US government would trade immediate comfort for long-term survival of the species, I don't think other governments would have the same priorities. I'd assume China would be one of the first to build multiple O'Neil cylinders with carefully controlled environments and as little Earth-originated biomass as possible (to contain the blight).
Cooper sobbing as he listened to years of messages from his family? I wish I could've written that. The twist where Mann activated his beacon because he didn't want to die alone? Solid.
We need the gravity equation to get off Earth because the crops are dying? Love can escape a black hole through the tesseract? This is just patent technobabble nonsense.
Where _Interstellar_ fails is in exposition. Love is the most powerful force in the universe? The Fifth Element pulled that off by not pretending to be serious. Anything is permitted in science fiction, but once you commit to explaining it, you need to make sense.