Your comments

Here you can see the new keys in the additional row above the standard keyboard. You can swipe over the keys in the direction of the special character you want to enter. This allows you to quickly reach all special characters without the need to scroll.

The tab button also supports untab (to indent and unindent code), delete forward, go to line start and go to line end. 
 
I've just added basic support for symbolic links in Textastic 4.0. I will submit this version to Apple today.
You could try to use a hard link instead of a symbol link (don't use the "-s" switch).

That said, I don't support jailbroken devices - I don't even have one.
Possibly. But I've added support for Firebug Lite in Textastic 4.0. I'm going to submit this version to Apple today.
Note: it's just the @import directive that doesn't work for local files. If you enter a complete url like
@import "http://www.example.com/test.less"

then it does work without modification.

Unfortunately, there is not much I can do about it. This would need to be changed in LESS, so that it doesn't hard-code supported schemas but checks if the path starts with <schema>:/

The problem is in the "loadStyleSheet" function of LESS: https://github.com/cloudhead/less.js/blob/master/dist/less-1.2.2.js#L3079


Textastic uses the custom "preview://" protocol, not "file://". This is needed to work around a caching but in iOS 5. 


So if you the change regular expression in this line:

if (! /^(https?|file):/.test(href)) {

to

if (! /^(https?|file|preview):/.test(href)) {

then it works for me in Textastic.

The basic sample works for me as described in my last comment.
I've just tried it out and it works fine for me:


test.html:

<!DOCTYPE HTML>

<html>
<head>
<meta charset="utf-8">
<title>Less Test</title>
<link rel="stylesheet/less" href="style.less" type="text/css" />
<script src="lesscss.js" type="text/javascript"></script>
</head>
<body>
This is a test for lesscss. 
</body>
</html>

style.less:

@color: blue;

body {
color: @color;
}

I've put the lesscss.js in the same folder as text.html and style.less. This resulted in blue body text.

Can you please explain in more detail what you are trying to do? 

You can access your Dropbox using a remote connection as explained in the manual: http://www.textasticapp.com/v3/manual/lessons/How_do_I_access_files_in_my_Dropbox_account.html
Due to sandboxing restrictions in iOS, an app can only access its own files. So this won't work, unless I'm missing something.