The argument to a 'slippery slope' lies precisely in the inevitability which is claimed for the final undesirable outcome. Once you step onto such a slope its slipperiness is so great that you cannot achieve the traction or friction which would be needed to stop at some point along the way: you must continue all the way to the bottom.
In 176, "All these things could happen, but it is hard to prove that they will happen" is an argument against the slippery slope: the end is not inevitable, the slope is not so slippery that we cannot stop at some point short of the bottom.
In 177, "decision two" represents the bottom of the slope and "decision one" its top. Even though it "was not thought to follow from decision one when decision one was made", it did in fact "follow from it after all". Once decision one was made, decision two followed inevitably: that is "a true slippery slope".
"Increased X%" or "increased by X%" or "up X%" or "up by X%" all mean the same. To increase an amount by a percentage is to take the original amount, and add to it the given percentage of that amount. Thus 'increased by 100%' means the same as 'doubled'. 100 increased by 120% is equal to 100 + (100 x 1.20) which is 220.
Percentage change calculator
It is common to use everyday fractions for some percentages, e.g. if something is increased by 25, 33.3, 50, or 75 per cent we can say it is increased by a quarter, a third, a half, or three quarters respectively.
In your question, you write:
My understanding is:
If our 2019 sales is up 120% (or has increased 120%) from year 2018,
it means, for instance, US$ 100 million in 2018, and US$ 120 million
in 2019
Your understanding is not correct. if your sales were $100 million in 2018 and $120 million in 2019, then they are up 20% (or have increased 20%) from 2018.
Best Answer
"Sleep on someone" is an informal phrase; some might consider it a slang. It basically means to ignore someone for whatever reason.
Note this is different from "sleep on it", which means