Leaflet / Google Map baselayer / markers not visible

Just a quick fix: you were on the right track with the z-index. Try adding this to your own css file (may break other things! I simply got it to display the markers):

.leaflet-map-pane {
    z-index: 2 !important;
}

.leaflet-google-layer {
    z-index: 1 !important;
}