1. I would say they both have a message of possibility, what could be. In contrast, "I Dwell in Possibility" is looking beyond the black and white of life, look for the color and all the opportunities beyond. "Do Not Go Gentle" is a persuasion to fight with all you've got, to not go easily, don't slip away.
2. I DWELL IN POSSIBILITY: The poem is written in iambic tetrameter. The beginning of the poem follows the rules of this form very well, then gets crazy by the end. Compare this to poetry: at first a poem is just black and white words on paper but while reading it can offer limitless opportunities to the imagination.
DO NOT GO GENTLE...: The form is villanelle, iambic pentameter. Do not give in, the words say. Fight with all you've got. This message is repetitious throughout the poem, which repetition is found in iambic pentameter.
3. Together, content and form make meaning, which is the message the poet gives to the reader.