The answer to the first part of When did planetary scientists realize Venus' surface pressure was almost 100x that on Earth? How did they find out? is 1967, though it was known or at least suspected to be substantially denser than Earth's for a while before that.
The answer to Launch to orbit delta-v penalty from Venus compared to Earth? will probably turn out to be some expression like "huge".
Has a hard science fiction story ever addressed a way to launch to space from the planet Venus using conventional rocket propulsion, describing how its very dense atmosphere was managed, especially after it was known that Venus' atmosphere has a density of about 7% of water in 1967† Either a really, really powerful rocket, or perhaps using a high altitude aircraft or balloon to get above most of the atmosphere?
†According to this answer several years before the actual measurement in 1967 Venus' atmosphere was already suspected to be very dense.