You can start the Photos App using the ApplicationActivationManager. Refer to https://stackoverflow.com/questions/12925748/iapplicationactivationmanageractivateapplication-in-c.
Adapting the code in the above SO answer, the following C# .Net Framework console application will start the Photos App with a file and wait for it to be closed. Error checking has been omitted.
static void Main(string[] args)
{
Guid clsidActivationManager = new Guid("45BA127D-10A8-46EA-8AB7-56EA9078943C");
Type ta = Type.GetTypeFromCLSID(clsidActivationManager);
IApplicationActivationManager iAppMgr = (IApplicationActivationManager)Activator.CreateInstance(ta);
uint pid;
IShellItemArray array = GetShellItemArray(@"C:\Users\RLWA32\Pictures\propget.png");
iAppMgr.ActivateForFile("Microsoft.Windows.Photos_8wekyb3d8bbwe!App", array, "Open",
out pid);
Console.WriteLine("Photos App activated for file");
Process Photos = Process.GetProcessById((int)pid);
Photos.WaitForExit();
Console.WriteLine("Photos App has exited");
}