Хотя день и был длинным, я всё-таки дочитала письмо до конца и наконец поняла, что от меня хотели.

No. Here и is not the normal coordinating and.

The pattern хотя ... и ... is a common concessive structure meaning although / even though. The и adds emphasis, something like even though or although indeed.

So:

• Хотя день был длинным = Although the day was long
• Хотя день и был длинным = Although the day was long / Even though the day was long

The sentence would still be grammatical without и, but with и it sounds a bit more natural and expressive in this kind of contrast.

Длинным is the instrumental singular form of длинный.

After был (was), Russian often uses the instrumental case for the predicate adjective or noun, especially in more neutral or written style:

• День был длинным = The day was long

So here:

• день = masculine singular
• длинным = instrumental masculine/neuter singular

You may also see nominative in some contexts, but был длинным is a very standard and natural choice.

Дочитала comes from дочитать, a perfective verb meaning to finish reading something, to read through to the end.

This is more specific than:

• читала = was reading / read (imperfective, process)
• прочитала = read (perfective, completed action)
• дочитала = finished reading, with a strong sense of reaching the end

So дочитала письмо suggests that the speaker had been reading the letter and finally got all the way through it.

That fits the sentence well, because the next result is и наконец поняла — only after finishing the letter did she finally understand.

Because Russian often uses both together for emphasis and clarity.

• дочитала письмо = finished reading the letter
• дочитала письмо до конца = finished reading the letter all the way to the end

So до конца is not strictly necessary, but it reinforces the idea of reaching the end. It can also make the sentence sound a bit fuller and more expressive.

This kind of doubling is very common in Russian:

• дослушать до конца
• досмотреть до конца
• дочитать до конца

Всё-таки means something like:

• still
• nevertheless
• all the same
• after all

In this sentence it signals that the action happened despite the difficult circumstance mentioned before:

• Хотя день и был длинным = Although the day was long
• я всё-таки дочитала письмо = I still / nevertheless finished the letter

So the idea is: the day was long, maybe she was tired, maybe it was hard to keep going — but she did it anyway.

In the past tense, Russian verbs agree with the subject in gender and number.

With я, the gender is only clear from context or from the verb form itself:

• я дочитал, я понял = a male speaker
• я дочитала, я поняла = a female speaker

So this sentence tells us that the speaker is female.

This is a very common Russian structure. The plural verb is used in an indefinite-personal way, where the people involved are not named because they are unknown, unimportant, or obvious from context.

So:

• что от меня хотели does not necessarily mean a specific they
• it means something like what was wanted from me or what they wanted me to do

Russian often uses 3rd person plural this way where English might use:

• they
• someone
• a passive construction

For example:

• Меня попросили прийти = I was asked to come
• Мне сказали подождать = I was told to wait

Here что от меня хотели is very natural and idiomatic.

Меня is the genitive form of я, because the preposition от takes the genitive case.

So:

• я = I
• меня = me / of me in certain case contexts
• от меня = from me

The expression хотеть от кого-то means to want something from someone or to expect something of someone.

So:

• что от меня хотели = what they wanted from me
• more idiomatically: what they expected me to do / what was wanted of me

They do different jobs.

Всё-таки expresses concession:

• despite the long day, she still finished reading

Наконец expresses finally, usually after delay, difficulty, or gradual progress:

• she finally understood

So the sequence is:

1. The day was long.
2. Even so, she kept going and finished the letter.
3. Then she finally understood what was wanted from her.

If you remove one of these words, the sentence loses part of its nuance.

There are two commas because there are two subordinate clauses.

1. Хотя день и был длинным, ...

   The clause introduced by хотя (although) is subordinate, so it is separated by a comma from the main clause.

2. ... поняла, что от меня хотели

   The clause introduced by что (that / what) is also subordinate, so it is separated by a comma.

So the structure is:

• subordinate concessive clause: Хотя день и был длинным
• main clause: я всё-таки дочитала письмо до конца и наконец поняла
• subordinate content clause: что от меня хотели

Yes. Russian word order is flexible, but changing it changes the emphasis.

The given order is very natural:

• Хотя день и был длинным sets the background first
• я всё-таки дочитала письмо до конца gives the action achieved despite that background
• и наконец поняла gives the final result

You could rearrange parts, but the focus would shift. For example:

• Я всё-таки дочитала письмо до конца, хотя день и был длинным...

  This puts more immediate focus on I still finished the letter.

Russian word order is often about information structure and emphasis, not just grammar.

It is the accusative singular, because it is the direct object of дочитала.

• дочитала что? — письмо

Since письмо is an inanimate neuter noun, its accusative singular looks exactly like the nominative singular:

• nominative: письмо
• accusative: письмо

So the form does not change, but the function is different.