I haven't had a chance to check the authentication (question 1) but on question 2, you were 100% right on the entry issues. I wasn't filtering the results of get-vmguestnetworkinterface and it was reporting:
VM Name IP IPPolicy SubnetMask
-- ---- -- -------- ----------
Mockup *******... Local Area Connection 192.168.0.249 Static 255.255...
Mockup *******... isatap.{43E80AEF-EA33-... Static
Mockup *******... Local Area Connection* 11 Static
adding | where { $_.Name -eq "Local Area Connection"} cleared up that issue.
when I get a chance I'll check the authentication as you recommeneded. I'm not so sure on those results as it does seem to pass the credentials correctly. Question 1 might be more of a Microsoft question.
Thanks for the help!
Modified code in case anyone stubles across this later:
$interface = Get-VMGuestNetworkInterface -VMGuest $guest -GuestUser administrator -GuestPassword password | where { $_.Name -eq "Local Area Connection"}