Yes, that would be possible to implement, but isn't yet available in the current version of Textastic.

You need to create a folder called "#Textastic" (with a capital "T"!) in Local Files yourself.

You can find more info with screenshots in the manual at http://www.textasticapp.com/v5/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html

In general, the auto-correct feature in iOS does not work with hardware keyboards, but only works with the virtual on-screen keyboard. You can try this in any other app like Apple's Pages or the iOS Notes app.

Fortran is already supported. It is also listed as a supported language at http://www.textasticapp.com/v5/manual/lessons/Which_file_types_are_supported.html

This sounds like a useful addition. I don't have plans for an Android version of Textastic.

Actually, the preview functionality is meant for HTML, CSS, JavaScript and Markdown files, so it's pure luck that it displays something meaningful for CSV files.

Internally, Textastic uses a regular UIWebView, which is basically the same WebKit engine that is used by Mobile Safari. It looks like WebKit does interpret the CSV file and displays it as a table. Unfortunately I don't know what exactly it supports, but from the screenshots it looks like it ignores the "sep=" option and instead tries to interpret the file on its own.

Text view commands like cursor movements or page up/down is completely handled by the iOS text input subsystem. If you try it in Pages for example, you will see that it doesn't support page up/down either (correct me if I'm wrong).