While writing using the third person, is "we" appropriate?

You can write in first or third person, depending on whether you want to give off an impression of being distant from the "you" in the sketch. Write two versions...

Unless the letter contains personal information, it is appropriate to let others read it, as a Christmas letter is an extended form of a Christmas card.

Either forget it, or try and include the idea of that paragraph in other paragraphs. If you have the opportunity to edit the essay before submitting it, you can just...

One should write in first person point of view, as the reader wants your experience and thoughts of the passage that was given to you.

Yes, it is absolutely fine.

Allows the author to divulge information otherwise unknown to the reader. If in the context of a piece of professional analysis or otherwise, it makes the writing more professional.

No, it's not necessary. You can format your text any way you like. Just make sure to format all items of a category the same way. If you use quotes,...

Write it in the third person.

Write on another sheet of paper, then read it over to make sure everything is correct before transferring the writing to your diary. Or, just use pencil so you can...

If you would like to write long stories instead, go for it. You could also just write some scenes (dialogues, battles, descriptions, etc) for a long story. Skills will come...