Q1: How can I do a export of keywords with present version already today?
You can use the File>ExportGridFields tool to export the keywords column to an external .csv or .txt file.
Then you can post-process that file, using  a script, etc., to build your list of sorted, an unique, keywords list.
Here is a simple script you could use to post-process that exported keywords column file.
/*****************
Helper prototype methods
*****************/
if (!Array.indexOf) {
  Array.prototype.indexOf = function (obj, start) {
    for (var i = (start || 0); i < this.length; i++) {
      if (this[i] == obj) {
        return i;
      }
    }
    return -1;
  }
}
String.prototype.trim = function() {
    var    str = this.replace(/^\s\s*/, ''),
        ws = /\s/,
        i = str.length;
    while (ws.test(str.charAt(--i)));
    return str.slice(0, i + 1);
}
Array.prototype.uniqueMerge = function( a ) {
    for ( var i = 0, l = a.length; i<l; ++i ) {
    var s=a[i].replace(/['"]/g,'').trim();
        if (s && this.indexOf( s ) === -1 ) {
            this.push( s );
        }
    }
    return this
};
/*****************
code starts here
****************/
var fso = new ActiveXObject("Scripting.FileSystemObject");
f = fso.OpenTextFile(WScript.Arguments.Item(0), 1);
var keywordsList=new Array;
while (!f.AtEndOfStream) {
   var keywords=f.ReadLine().split(/,|;/);
     keywordsList=keywordsList.uniqueMerge(keywords);
}
keywordsList=keywordsList.sort();
WScript.echo(keywordsList.join('; '))Just save this code to a BuildKeywordsList.js file, and drag-drop on it the PDFE exported keywords column file. It will show you a popup with the sorted list of all your PDFs keywords.
if you want that output in a file, run the next command line in the same folder where you have the BuildKeywordsList.js and Keywords.txt
cscript //NoLogo BuildKeywordsList.js Keywords.txt>KeywordsList.txtAnd feel free to ask, If you have doubts on the usage of this solution.
Q2: What is your opinion if such an export is worth considering ? And if yes, what are your plans? 
The next to release PDFE version has a scripting tool, so custom scripting this kind of tasks will be quite easy.
Q3: Do you know other software permitting an export of keywords after scanning pdf´s? 
No