for item 1
https://getmusicbee.com/patches/MusicBee36_Patched.zip
use
<element id="Panel">
<images stretchY1="1" stretchY2="1"
default="background.svg"
/>
<images category="Panel.Left"
default="panelleft.svg"
/>
<images category="Panel.Right"
default="panelright.svg"
/>
</element>
for 2 & 3, there is no flexibility in the skin creator tool to do that and I cant see myself rewriting it
i must be missing something here because i am struggling to see why the difficultly.
Here is an example with .png and all images are in a sub-folder. You probably already do something similar for the TrackInfoPanel element
<element id="Panel">
<images stretchY1="1" stretchY2="1"
default="images\background.png"
/>
<images category="Panel.Left"
default="images\panelleft.png"
/>
<images category="Panel.Right"
default="images\panelright.png"
/>
</element>