It would have been cool if such things had not occured on Venus, if Earth had a living twin.

Alas, at least Venus as it is now is pretty cool.

I mean, a planet whose day is longer than its year probably will get toasted. So pretty sure Venus was never a paradise

Ah, you know you've made it when an article you worked on for NASA winds up getting covered elsewhere - imitation is flattery, right? ;)

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Something happened to make the whole planet north roll south so that it ended with what we have now rotating backwards

maybe before its original rotation direction and period was ok

Is Venus upside down, or does it just spin backwards?

Also Venus is close enough to the Sun that it gets about 1.7 times as much radiation as the Earth. That means Venus's average temperature would be about 200 C ( 390 F ) hotter than Earth even if it wasn't a runaway green house. Venus was doomed to have all it's water boil off as the Sun heated up just because its so close to the Sun.

Being hit by something that flipped it upside down is one of several hypothesis for both venus and Uranus, other hypothesis are atmospheric tidal torques along being hit ......etc, etc, so yea there is a chance that something did flipped it upside down and messed its rotation, but we don't know enough yet to confirm the actual cause

If Venus today was Earth-like, fast rotation would’ve killed it.

Slow rotation means not a lot of wind to dissipate clouds forming on the dayside. That helps cool the planet down massively. I recall some works suggesting that Venus could’ve maintained habitability if we terraform it today and left it alone like that.

Both are the same, it just depends on how you define 0 degrees tilt. If you define it as the direction of rotation (north pole is the pole where the planet pins counter clockwise when you look down on it), it’s upside down. If you define it by direction (north pole is the pole that points in the same direction as Earth’s), it spins backward.

Surface temperature doesn’t scale linearly with irradiance.

You may end with same results but it's not about definition, its the actual event

did an impact (or impacts) flipped the planet 180 degrees or did it made it rotate the opposite direction? We can argue that Uranus ended sidewise 90 degrees so flipping is a compelling possibility but we don't have yet enough data to show a definitive answer

I’m responding to the other person.

A lot of things in planetary science can be handwaved away with impacts, that doesn’t mean it’s the correct answer for everything. Tidal-locking, atmospheric tides, rotation instability at very low rotation rates, are also present.

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Venus' modern retrograde rotation is most likely the product of atmospheric braking combined with a slow initial rate of rotation.

However, slow rotation and even outright tidal locking does not necessarily rule out long-term habitability. Oceans and atmospheres can transfer absorbed energy, for instance. In "Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate", Yang et al. argued that a slower rotation rate is beneficial the closer a planet is relative to its host star.