OpenFileDialog default path

It seems like all you need to do is the following:

string path; // this is the path that you are checking.
if(Directory.Exists(path)) {
    openFileDialog1.InitialDirectory = path;
} else {
    openFileDialog1.InitialDirectory = @"C:\";
} 

That is unless I'm missing something.

Where is the last used directory saved?

It is stored in the registry. The exact location depends on the Windows version, for Win7 it is HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32. A quick look with regedit ought to convince you that you don't want to mess with that.

The simple workaround is to provide a valid path. If the one you calculate isn't valid, Directory.Exists returns false, then provide a valid one. Like the Documents folder returned by Environment.GetFolderPath(). Then again, nothing wrong with the last used one either, the user will easily recognize it with good odds that it happens to be close to the desired one.

I don't think there is anything built in for that. Just check before you open the dialog:

if (!Directory.Exists(initialDirectory))
{
    openFileDialog1.InitialDirectory = @"C:\";
}

Check to see if the ExcelFilePath exists, you check to see if it's null or empty, however if before your block you check to see if the directory exists, and if it doesn't reset the value to an empty string you should be golden.

(yes you'll need to apply your file name logic etc earlier) however once you've parsed all of that out, it's trivial to determine if the directory exits

if (!Directory.Exists(excelPath))
{
    ExcelFilePath = String.Empty;
}