Two major theme that emerge from The Selected Poems of Langston Hughes are: (1) survival in the face of adversity; and ....(2) the continuum of...

There is no way this question can be answered in the space given here.  I can identify several of his poems and how they fit the themes.  You might have to go back to the poems and find the lines that substantiate it.  In these poems, you might have to address some of the other questions you raise.

The theme of survival in the face of adversity can be seen in so many of his poems, but I think these are fairly powerful:

"The Negro Speaks of Rivers"

"Mother to Son"

"Let America Be America Again"

"Daybreak in Alabama"

In these poems, you see the element of resiliency and strength through overwhelming challenges.  In these poems, the speakers are ones that are not overwhelmingly happy, but they are real and vibrant.  Their experiences in life contribute to their emotional and mental austerity and they will not be denied.  In these poems, the speakers make reference, directly or implied, that they are a part of something which gives them strength and the ability to persevere.  This capacity for endurance which will invariably result in triumph could be something spiritual or the link to being of ancestral Africa, a source of harnessed energy.  The speakers in these poems feature a love of life, which is why they experience what they do.  They have not quit in the face of a challenging consciousness, as they understand that this is their lot to bear and with broad shoulders, they seem to do so.  Whether it is passing lessons down to younger generations, or speaking to the reader, the speakers in these poems feature a love that gives them strength.  They do not strike me as bitter individuals that have been denied love.  In terms of "The Negro Speaks of Rivers" and music, I think one can trace the geographic change in what the speaker has seen to the types of music present in each of those areas.  The music in these areas reflect the different experiences of Africans.  From music indigenous to Africa, to the music in Egypt, to the music along the Mississippi, to the tonal qualities of music in the North, where "Mr. Lincoln" resides, the African's experience in the poem can be linked to different musical expressions of identity.