Dunno 'bout question 1, but a thought about #2:
Although it might be useful in some cases to have resolution specific scene layers this is a thing that imo should be avoided at all costs to keep levels generic.
There are so many different resolutions, you don't want to (and shouldn't!) include every single one of them to make your level playable. What about some guy with a strange resolution you didn't even think about?
As I said, just a thought. If there's an elegant way to set for example a default or "fallback" setting etc. I could live with it (if it's unavoidable for the level to "work").

It is about my first mode http://goofans.com/download/theme/new-look-world-of-goo. Can you test it?

It works in normal resolutions but not in widescreen, you can see the end of the image and the right and left in widescreen.
If I set the scale to match the widescreen it would be to wide for the normal resolution.
That isn't the case with the original blue bg.phg and they are bouth the same proportion!!

(the image is in high resolution 2200x2200).

Currently it is working by overriding .xml but I want it to merge.

I wasn't thinking about supporting all resoulutions, just those that GooTool supports.

Interesting, I thought that for widescreen the visible area is just cropped and the width is left untouched. Read that somewhere and it makes sense to me as one can actually see way "less" (regarding height) in widescreen. :I
I can try your mod this evening, I'm currently at work. Unfortuanetly no WoG here.

There was a bug, looked horrible!!! I fixed it now. Try downloading the 1.1 version

Took a look at your mod, but didn't have the time to do any widescreen testing (except for looking at it).
Just had a thought about your first question, though.
You have an error in the matching pattern for the BallInstance you want to replace (and the tag syntax itself for that matter).
Again, no time to test, but I think this goes in the right direction:

<!-- delete old BallInstance -->
<xsl:template match="/level/BallInstance[@id=3]" />
 
<!-- insert water goo with same id -->
<xsl:template match="/level/comment()[contains(., 'Balls')]">
  <xsl:copy />
  <BallInstance type="water" x="-122.63" y="284.03" id="3" angle="0" />
</xsl:template>

Yeah.. I was just beginig to understand... Used merge in Loadizing Mod and it works good. Thx for all your troubles.

   <!-- Delete only one goo -->
   <xsl:template match="/level/BallInstance/id="3" "/> , <xsl:template 
 
   <!-- insert only one water goo -->
   match="/level/comment()[contains(., 'Balls')]">
    <xsl:copy/>
 
    <BallInstance type="water" x="-122.63" y="284.03" id="3" angle="0" />
 
   </xsl:template>
 
  </xsl:template>

Something like this should do the trick:

  <!-- Replace the goo with id 3 -->
  <xsl:template match="/level/BallInstance[@id=3]">
    <BallInstance type="water" x="-122.63" y="284.03" id="3" angle="0" />
  </xsl:template>

No, I don't think so. The only widescreen-specific stuff I'm aware of in the XML is the camera positioning in a level.

GooTool offers up every resolution supported by the user's graphic card. Whether WoG runs under that resolution is at the user's risk!

-davidc