A few comments on the gsettings schemas.

 path="/desktop/gnome/peripherals/keyboard/">

The recommended convention is for paths to be of the /org/gnome/foo/bar style.
(same goes for all the other schemas too.)

    <key name="default-group" type="i">
      <default>-1</default>

Should maybe have a <range />.

    <key name="x" type="i">
      <default>-1</default>
      <_summary>The Keyboard Preview, X offset</_summary>
      <_description>The Keyboard Preview, X offset</_description>
    </key>
    <key name="y" type="i">
      <default>-1</default>
      <_summary>The Keyboard Preview, Y offset</_summary>
      <_description>The Keyboard Preview, Y offset</_description>
    </key>
    <key name="width" type="i">
      <default>-1</default>
      <_summary>The Keyboard Preview, width</_summary>
      <_description>The Keyboard Preview, width</_description>
    </key>
    <key name="height" type="i">
      <default>-1</default>
      <_summary>The Keyboard Preview, height</_summary>
      <_description>The Keyboard Preview, height</_description>

These should probably all have a <range />, like e.g. -1 .. maxint16 ?

  <schema gettext-domain="@GETTEXT_PACKAGE@" id="org.gnome.libgnomekbd.indicator" path="/desktop/gnome/peripherals/keyboard/indicator/">
    <key name="secondary" type="i">
      <default>0</default>

I guess negative numbers don't occur here, so maybe use "u"? Or a <range /> with min=0.

    <key name="font-size" type="i">
      <default>10</default>

Needs a <range />.