1) The wedding guest hadn't chosen to hear the mariner's tale
2) The storm changes the course of the ship (it it correct to use "course") and is represented as a huge bird (is as correct?)
3) The flowers help (better enable??) the poet to think about his past
4)Nature is endowed with a spirit whose presence is manifest not only in living beings but also in inanimate objects (are the two last words correct?)
5)Nature is described according to (or on the basis of? ) a pantheistic view
6) He could fascinate others (or the others) with his eye
7) The sight (and not the view?)of the flowers inspired (or infused?)joy in the poet / instead of "filled the poet's heart with joy"
8) The flowers are made alive ( or are described as though they were alive???)
9) The poet imagines them to be ten thousand in number (can you also say: imagines they are ten...)
Thanks
1. If there's only one guest, this is correct. If there are many guests, don't forget to make the word plural!
2. Correct.
3. Fine as is -- "help" is good.
4. Wordy -- delete "whose presence is" and put a comma after "spirit." The rest is fine.
5. "according to" is fine.
6. "others" is fine.
7. Ignore what you put in parentheses; it's fine as is.
8. This time I'd use the words in parentheses. How will it read?
9. "imagines they are" is smoother wording, yes.