If I'm going to print a single generic answer sheet, how is it supposed that the teacher's answer sheet will have a distinctive number?

When using the photocopy allowed mode (you choose it when selecting the first scans to process), AMC assigns a copy number to each scan, in addition to the student/page id printed at the top of the page.

Is there an option to create an answer sheet for post correction that looks as great as answer sheets created with their respective exams, but using AMC-TXT sintaxis?

Not yet. This is quite a long job to write the code that will produce a LaTeX file simply for a large set of needs (for example allowing to have multiple columns at the top as for you the Curso and Asignatura one), different number of questions, of answers for questions, different layouts, etc.

Finally: if I create an exam using AMC-TXT and then I edit the DOC-filtered.tex file, will AMC be able to perform the correction process properly?

Yes. Simply copy the DOC-filtered.tex to for example source.tex, edit it and tell AMC that the source file is now source.tex.

2.1¶

You have to adapt the \AMCcode code: near \begin{document},

\def\CursoCode{ %
{\def\AMCbeginQuestion##1##2{}%
\AMCnobloc\setlength{\parindent}{0pt}%
\def\AMCbeginAnswer{\hspace{0pt}\vbox\bgroup}%
\def\AMCendAnswer{\vspace{-\AMCcodeVspace}\egroup}%
\def\AMCanswer##1##2{\hbox{##1}\vspace{\AMCcodeVspace}}%
\begin{question}{class}\QuestionIndicative%
\begin{choicescustom}[o]\scoring{auto=1}%
\wrongchoice[7]{7}
\wrongchoice[8]{8}
\wrongchoice[I]{I}
\wrongchoice[II]{II}
\wrongchoice[III]{III}
\end{choicescustom}%
\end{question}}%
}

and then replace \AMCcode{class}{1} with \CursoCode. However, the III in the last circle seems a little drak for me, and I'm afraid this will lead to false positives. You can prefer something like that:

\def\CursoCode{ %
{\def\AMCbeginQuestion##1##2{}%
\AMCnobloc\setlength{\parindent}{0pt}%
\def\AMCbeginAnswer{\hspace{0pt}\vbox\bgroup}%
\def\AMCendAnswer{\vspace{-\AMCcodeVspace}\egroup}%
\def\AMCanswer##1##2{\hbox{##1 ##2}\vspace{\AMCcodeVspace}}%
\begin{question}{class}\QuestionIndicative%
\begin{choicescustom}[o]\scoring{auto=1}%
\wrongchoice[7]{7}
\wrongchoice[8]{8}
\wrongchoice[1]{I}
\wrongchoice[2]{II}
\wrongchoice[3]{III}
\end{choicescustom}%
\end{question}}%
}

2.2 & 2.3¶

Use this modified \AMCbeginQuestion:

\def\AMCbeginQuestion#1#2{\vspace{2ex}\par\noindent\hbox to 3em{\hspace*{\fill}\bf #1 \hspace*{1em}}}

I think you should also add some space between answers:

\def\AMCanswer#1#2{#1\hspace{.5em}}