Kitzur Shulchan Aruch 141:15 says you should not read along in the Megillah if you don't have a kosher scroll, because even if you can simultaneously concentrate on the public reading, people nearby might inadvertently listen to you instead of the reader:
מִי שֶׁיֵשׁ לְפָנָיו מְגִלָּה פְסוּלָה אוֹ חֻמָּשׁ, לֹא יִקְרָא עִם שְּׁלִיחַ הַצִבּוּר. כִּי אִם הוּא קוֹרֵא, אֵינוֹ יָכוֹל לְכַוֵּן לִשְׁמֹעַ מִן שְּׁלִיחַ הַצִבּוּר. וַאֲפִלוּ אִם הוּא יְכַוֵּן, שֶׁמָּא יִשְׁמַע אַחֵר מַה שֶׁהוּא קוֹרֵא, וְלֹא יְכַוֵן לִקְרִיאַת שְּׁלִיחַ הַצִּבּוּר
But in halacha 13, we learn that the reader must have in mind to fulfill the obligation of all the listeners, and vice versa:
הַקוֹרֵא אֶת הַמְּגִלָּה, צָרִיךְ לְכַוֵּן לְהוֹצִיא אֶת כָּל הַשּׁוֹמְעִים. וְגַם הַשּׁוֹמֵעַ צָרִיךְ לְכַוֵּן לָצֵאת
If so, wouldn't it be problematic to read aloud even from a kosher scroll? Presumably you don't have the other congregants in mind, and they don't intend to listen to you, so anyone overhearing your reading would fail to fulfill the mitzvah.
I suppose this could be one reason it's best to read along silently, but in that case why tie the issue to the lack of a kosher scoll?
