>> - when the current line after the prompt is too long the prompt 
>> disappears
>
> Yes, set visual-line-mode -1 for this. Is the only solution so far and 
> what I will do in the setup probably.
>

No, it is not the only solution, my code handles that case without using 
visual-line-mode.  By the way, I just checked it (with your code) and 
apparently enabling visual-line-mode does not solve that problem.

>
> Did you applied the patch cleanly?
>

As far as I can see yes.  I already tried your code twice, did you try 
mine?  I attach the improved version which handles the minibuffer-only 
case.

Anyway, IMO you not are using the best approach on this project, you are 
trying to do too many things at once.  Completion mechanisms are a very 
complex thing, and I believe the best way is to do small incremental 
improvements with minimal changes, and to test them thoroughly before 
implementing the next improvement.