And there I was thinking you were a student. I'm sorry. It's kind of you to help a student with a subject you don't teach.
Hm, let's see. The two final questions are dependant on each other. Whatever "letters" we'd pick out as repeated would define the answer we give. Perhaps, if I'd be a bit more systematic, it could be like this:
What letters have been repeated in the last stanza?
1. Any English text that contains more than 26 letters must repeat at least one letter. Some letter-repetitions are a function of the language itself. I shall therefore look not for all repetitions, but for "meaningful repetitions", i.e. repetitions that either follow a pattern, or are conventional poetic tools (such as rhymes or alliteration).
2. Since most poetry works best when read aloud, I am going to look for repeated "spoken letters", rather than for "written letters". For example, the letter "o" is written down a lot, but since it sounds different in "wood", "lovely" or "promises", I will not count it as a repetition (meaningful or otherwise). Similarly, since the two o's in "wood" represent a single sound, I will treat the "oo" as one letter only.
The most obvious repetition in the stanza is the end-rhyme "-eep", or rather [i:p]. All instances of "ee" (or [i:]) are in the last syllable. The letter "p" concludes each line, and is only found once within the lines: at the beginning of the word promise.
Line one contains an alliteration. Two words start with the letter "d". Three of the four d's that occur in the last stanza are in the phrase "dark and deep" (the remaining one being in "woods"). Two of them are at the start of a word, and one concludes "and". Whether the d in "and" is articulated or not depends on the reader of the poem. Leaving out the d in "and" (or rather conflating it with the d in "deep") enhances the alliterative effect.
Finally, in lines two, three, and four the letter "i", as [ai], is repeated in prominent position. In both lines two and three the "i" is the vowel in the second syllable; in line three it is also the vowel in the next to last syllable. (The fourth line is an exact repetition of the third line, so naturally the "i"-pattern of the third line is repeated as well.)
Why have they been repeated?
The rhyme "i:p" has been repeated to contribute to the poems line-structure. The repetition of the "p" in the word "promises" helps the word stand out as a key word. This effect is enhanced by using an "I" sound, a shorter and darker version of the vowel [i:] from the rhyme, which is not repeated elsewhere in the stanza.
The alliteration "dark and deep" helps make the phrase a unit, standing together against "lovely". These three adjectives are used to describe the "woods", the only other word in the poem that contains a d. The phonetic contrast between "lovely" and "dark and deep" is analoguos to the semantic difference. "Lovely" is an evaluation, whereas "dark and deep" are merely descriptive. Since the word "woods" also contains a "d" it is the word "lovely" that stands out. This enhances the semantic difference: things that are "dark and deep" are not conventionally evaluated as "lovely".
Finally, looking at the [ai] repetition one notices that [ai] is not only a diphtong, it is also a word. And in three of the five instances the diphtong means exactly that. The repetition of [ai] in the word miles is not semantically meaningful, but since it occupies the same position in the third and fourth lines that the word "I" occupied in the second, it can be read as a feint echo of the word, emphasising that the poet - who last explicitly appeared in the first line, and implicitly in the first line of the second stanza (my little horse) - has returned to his poem.
I'm not really sure this works, but it's the best I can currently do. It was a fun excercise. Perhaps you can tell that I like poetry?