Short answers, pulled from the story.
Mahatma Gandhi coined the term satyagraha as early as 1919. It is a Sanskrit compound of satya (truth) and agraha (polite insistence or holding firmly to), meaning truth-force or love-force. The word originated from a competition in the news-sheet Indian Opinion in South Africa in 1906, where Maganlal Gandhi first proposed the related word "Sadagraha" before Gandhi refined it.
Gandhi argued that satyagraha and passive resistance are fundamentally different in three ways: satyagraha is a weapon of the strong, it admits of no violence under any circumstance, and it insists upon truth at every moment. Passive resistance, as practiced in the West, had admitted of violence and was widely seen as a weapon of the weak.
Gandhi founded the Sabarmati Ashram to teach satyagraha and required practitioners to follow principles including nonviolence (ahimsa), truth, not stealing, non-possession, body-labour, control of desire, fearlessness, equal respect for all religions, and economic strategies such as boycotts of imported goods (swadeshi). For campaigns of civil resistance he published twelve additional rules of conduct.
Martin Luther King Jr. wrote that studying Gandhi's campaigns, especially the Salt March to the Sea and Gandhi's fasts, profoundly changed his thinking. He described satyagraha as the method for social reform he had been seeking and came to see the potency of love-force in social change. James Bevel's campaigns during the Civil Rights Movement were also shaped by satyagraha theory.
Gandhi held that means and ends are inseparable: the means to obtain an end are wrapped up in and attached to that end. He wrote "means are, after all, everything. As the means so the end," arguing that using unjust means to obtain justice will embed that injustice into the outcome. He illustrated this with the example of a watch: whether it is stolen property, your own property, or a gift depends entirely on the means used to obtain it.
No. In a September 1935 letter to P. Kodanda Rao of the Servants of India Society, Gandhi explicitly denied that his ideas of civil disobedience derived from Thoreau's 1849 essay. He stated that resistance in South Africa was already well advanced before he read Thoreau, and that he had coined satyagraha for Gujarati readers before encountering Thoreau's phrase. He later used Thoreau's terminology only to explain the struggle to English-speaking readers.