Too many things to ask together in one single question! I will give you the basic thematic pattern.
Frost's poem is about the choices in life--the crossrads we all have to encounter and then decide which is the way we want to pursue. Frost's traveller in the poem comes to one such crossing and tries to make a choice between two roads, the oft-trodden and the unusual which seems to be more virgin. There is a speculation about the comparative interests of the two roads and a decision to take one. The traveller does want to return to the road not taken at a later stage in life, he knows that roads lead on to more roads making it impossible to return.
As the traveller says at the end that whatever the road he has taken has had a shaping influence on his life and it has made all the difference. An ambivalence remains regarding the positive or the negative character of this difference, however.
There is an autobiographical aspect to the poem because it is also about Frost's efforts to publish himself from his native country and subsequent failure leading him to a decision of becoming an exile.