I had to edit your original question because it contained more than one question. Please remember that enotes does not permit you to ask multiple questions.
I personally think that the ending of the poem is very effective. Note how it captures the central theme of the inevitable passing of time, yet subtly changes it to support the initial plea of the speaker to his mistress in asking her to love him:
Thus, though we cannot make our sun
Stand still, yet we will make him run.
The speaker points towards the way that the course and path of the sun can never be halted. It is impossible to make it "stand still," but through making the most of every single moment and opportunity, it is possible to make the sun, that symbol of passing time and ageing, "run" in its course. The speaker thus challenges his mistress to try and live life as if they can elude time by "seizing the day" and not wasting the moment. A highly effective ending to this poem that attempts to make us see the importance of the here and now.