The original question had to be pared down. I advise you to ask the other questions in additional posts because they are very strong questions about both works. I think that the contrast between both poems is stark. Dickinson's poem speaks to the idea that there cannot be unity between the speaker and a lover. There is disunity evident in terms of the separation between both of them. The speaker is unable to construct a traditional religious understanding of Christianity as well as understand the love shared for "the other." The statement here is one in which there can only be the understanding of pain in this life caused by their separation, "despair."
This position is contrasted with the sense of unity in Silverstein's poem. The traditional view would be that very young and the very old are opposing forces, elements that could not reconciled because of their differences in life experiences. Yet, Sliverstein is able to constuct a narrative in which both understand one another through their shared understanding of marginalization. In this, Sliverstein alomst provides a counterpoint to Dickinson in suggesting that while the pain of consciousness is present, there can be acknowledgement of how ourselves and "the other" are similar. Dickinson's poem speaks to how unity with "the other" is not possible in this life, something that Silverstein seems to repudiate in his poem, suggesting that it is possible for individuals to find shared experiences with one another despite evident differences. This is where I see contrast in terms of each poem's approach to human companionship.