Some questions about UT best Practise and Isolator using

Hello, I have some questions about unit testing.
I have this very simple method (Share Point code):

 public Guid CreateNewSite(String siteUrl, string newSite)
        {
            if (!(string.IsNullOrEmpty(newSite) || string.IsNullOrEmpty(siteUrl)))
            {
                using (SPSite site = new SPSite(siteUrl))
                {
                    if (!site.AllWebs.Names.Contains(newSite))
                    {
                        return site.AllWebs.Add(newSite, newSite, "New Website", 1033, "STS#1", true, false).ID;
                    }
                }
            }
            return Guid.Empty;
        }

My Isolator test code is:

public void CreateNewSite_SiteNotExists_SiteGuidNotEmptySiteIsCreated()
        {
            //Arrange
            string webUrl = null;
            string webTitle = null;
            string webDescription = null;
            uint nLCID = 0;
            SPWebTemplate webTemplate = null;
            bool useUniquePermissions = false;
            bool bConvertIfThere = false;

            SharePointListCreator spListCreator = new SharePointListCreator();
            Guid webGuid = new Guid("371FCC0B-51A8-4DDC-884F-FD1A48CBCF52");
            var fakeSite = Isolate.Fake.NextInstance<SPSite>();
            Isolate.Swap.NextInstance<SPSite>().With(fakeSite);
            Isolate.WhenCalled(() => fakeSite.AllWebs.Names).WillReturn(new string[] { "nonExistsWeb" });
            Isolate.WhenCalled(() => fakeSite.AllWebs.Add(webUrl, webTitle, webDescription, nLCID, webTemplate, useUniquePermissions, bConvertIfThere).ID).WillReturn(webGuid);

            //Act
            Guid actual = spListCreator.CreateNewSite("http://local", "testWeb");

            //Assert
            Assert.AreEqual(webGuid, actual);
        }

1 question. Is it ok, that I test the methods return (Guid). I mean I test the contract of the method "CreateNewSite". If the site that should be createt not exist, and the parameters are not empty, the new site will be created and a Guid != Guid.Empty will be returned. Otherwise return Guid.Emtpy.

Or should I test, that the method site.AllWebs.Add(newSite, newSite, "New Website", 1033, "STS#1", true, false).ID; was called with the right parameters?

2 question.
Why I must create/ initialize the parameters for this method?:

Isolate.WhenCalled(() => fakeSite.AllWebs.Add(webUrl, webTitle, webDescription, nLCID, webTemplate, useUniquePermissions, bConvertIfThere).ID).WillReturn(webGuid);

In Ms Fakes this example looks like this:

Guid webGuid = new Guid("371FCC0B-51A8-4DDC-884F-FD1A48CBCF52");
                ShimSPSite.ConstructorString = (site, url) => { };
                ShimSPSite.AllInstances.AllWebsGet = (site) => new ShimSPWebCollection()
                {
                    AddStringStringStringUInt32StringBooleanBoolean =
                        (webUrl, webTitle, webDescription, nLCID, webTemplate, useUniquePermissions, bConvertIfThere) => new ShimSPWeb()
                            {
                                IDGet = () => webGuid
                            },
                    NamesGet = () => new string[] { "nonExistsWeb" }
                };

These parameters (webUrl, webTitle, webDescription, nLCID, webTemplate, useUniquePermissions, bConvertIfThere) are never declared or initialized. I think Fakes do this for me. But when the method AddStringStringStringUInt32StringBooleanBoolean is called, I can use these parameters for other things, like validate that this method was called with exact parameters I call the main method.
How can I do this in Isolator?
How can I unit test the void methods (interaction testing)?

I hope my post is not very confusing :)