First, an answer to the title's question, "Why do people sometimes confuse when to use will and would?":
The reason why English words are often confused one for another is that our grammar is complex.
When you compare English with many other languages, you will find that verbs, when conjugated, have very distinct forms. Not so with English.
Add on top of that, will/would are auxiliary verbs (as well as homonyms and homophones of nouns), so they conjugate with their verb.
The layers of understanding and knowledge needed to properly conjugate the auxiliary verbs and main verbs require thought and effort.
Some blame education, laziness, unwillingness to learn, and other factors for why grammar is not properly learned. (See YourDictionary.com Blog) Some are quicker to hear and then correct improper grammar, especially when they are accustomed to hearing proper English. Even monkeys have been shown to be able to do that. (See BBC News)
In the end, it's a complex question without a single answer.
And secondly, to answer (I corrected some issues with the sentence): "Does 'will' make your expression more emphatic and firm? 'Would' seems to lack that authority of 'will.' Does 'would add more flatness to the tone?"
As to tone, no. (See LiteraryDevices.com's piece on Tone) But what you're picking up on is that 'would' is a conditional verb. However, 'would' is not merely a conditional verb. It's used in many sections of the full and complete conjugation of 'will.'
A resource you may want to consider is located here: https://writingexplained.org/will-vs-would-difference