[trogggy]

I have written a login addon for Kodi, that attempts to change the homepage upon login, based on your credentials.

What it does is directly edit the kodi/userdata/addon_data/skin.xonfluence/settings.xml file upon login.

I can't seem to make it commit the changes.  When you make changes in the Skin Settings, they seem to take effect immediately.

Is there a function in the skin that commits the changes?  I can call it from within my addon.

You can't make changes to settings.xml while the skin is running by direct editing.  Either use json, or if you want to edit directly unload the skin, make changes and reload it.
edit: you can also use Skin.SetString to make changes while the skin is running - there are a few other skin-related built-in functions that also might be of use.  I did use the Skin Helper Service Skin Backup addon with xonfluence (to backup / restore settings.xml) at one point - it did the job.