Before the invention of Hangeul (한글), how difficult would it have been for Koreans to learn Chinese without a pronunciation guide?

ANSWER: Very difficult.

I have always suspected that the Korean language was originally just an old Chinese dialect instead of some unique language with unknown origins, as many scholars seem to suggest. There are just too many similarities between the Korean and Chinese languages. Moreover, Korea is just too close to China geographically to have been able to avoid the influence of the Chinese language over the past few thousand years. 

Anyway, the video below made me wonder how Koreans learned to pronounce the tens of thousands of Chinese characters before the invention of Hangeul (한글), the Korean alphabet. They would have had to either hear the Chinese pronounce the characters and then try to remember the thousands of different pronunciations, or they would have had to learn the meanings of the different characters and then assign their own pronunciations, something that I think would have been less likely. But what I am really curious about is how the Chinese came up with their pronunciations of the characters. 

Chinese characters are normally just one syllable. Does that mean that all Chinese words in the past were just one syllable? I kind of doubt it. Because even with the four Chinese tones, the Chinese would have had a hard time communicating with just one-syllable words. So, how did they come up with the pronunciations for the tens of thousands of characters they have? The way I think they probably did it was to take just one syllable of a multisyllable Chinese word and assign it to a unique character with the meaning of that multisyllable word. The Chinese would generally not have been able to understand the word by hearing just the one syllable, but they would have been able to understand the word if they saw the unique Chinese character assigned to that syllable. In other words, they would have been able to read it.

Anyway, all of this is probably explained somewhere by someone who knows what they are talking about, but I have not read anything about it, so all of this is just my talking out loud to myself.