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Hi,
thanks for asking - and please find the answers below:
 Is it possible to make Remote Buddy show a specified folder when 
pressing, for example, Menu-button. And start with that folder every 
time, not at last visited folder? I want this to be a general 
setting, for ALL apps.
  You can configure your own custom menu and add any folder of your 
choice to it via drag and drop from the Finder. From the online help:
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How do I build my own, simple, customized menu? 
• Locate the small Remote Buddy icon in your main screen's title bar. 
• Click on it and select Preferences from the dropdown menu. 
• In the Preferences window, click on Menu. 
• Now, in the bottom, left click on "+" to create and name your own 
profile. 
• You can now drag and drop menu elements from the left "Item 
Library" to the right: your new menu. 
-- cut --
If you always want to return to that menu, you can change the action 
associated to the Menu button to the respective action for that. From 
the online help:
-- cut -- 
How do I jump right to the main menu level when pressing the menu key? 
• Locate the small Remote Buddy icon in your main screen's title bar. 
• Click on it and select Preferences from the dropdown menu. 
• In the Preferences window, click on Mappings. 
• Select "global mapping" in the list on the right. 
• In the button/action list in the middle, click on the action "Open 
Remote Buddy menu" and select "Open Remote Buddy menu at main menu 
level" instead. 
-- cut --
The combination of a custom menu and the remapped menu button will 
give you what you are looking for.
 Can Remote Buddy show full file names, even when they're long?
  Not right now.
 Can I specify a certain file to always open with, for example VLC, 
so that I don't have to chose that each time?
  Remote Buddy will use whatever you set as default application for a 
file type / file in Finder as the first entry with the apps you can 
choose from. Thus, please use the Finder to set the default app for a 
file / file type - you'll just need to "double click" then.
If movies is what you are after, search for "movie library" in the 
forum for a how to to add non-DVDs to the movie library.
Best regards, 
Felix