How to Skip "Checking for Updates" during win 8.1 Install

I don't think using the /unattend command in that situation is the right way to do this.
During audit-mode, a lot of the post-user-creation stuff has still yet to be set up.
I think for your particular situation wanting to both use audit-mode and set a full fledged user's start menu, it's a bit more complicated.
First, you need to do the audit-mode without unattend, delete the administrator user folder and capture the drive.

Then you need to re-logon to the account and create a sample user. This will be your xml export method.
A lot of the start menu stuff is not set up properly for Administrator so you need to create an actual OOBE user.
Then use the Powershell command that Mr Jinje posted to export start layout. Name it something easy like c:\windows\setup\start.xml
Create a group policy making start.xml the default start screen layout.

Go into registry and export the key that sets the start.xml in gpedit.
Load reg hive on the offline hive from your capture to something like \Test
Change the reg and add the \Test part.
Import the new modified reg file so that your offline hive will now want to load start.xml from group policy
Now export the reg \Test stuff, and copy the start.xml to the offline image using dism mount in the same folder you set it to with group policy.

THEN, you should be set.

 

Well you could do that, but if you do it with a sample account you have to go and remove the user stuff from the registry. the audit mode version doesn't set up a user account, so it doesn't add those registry settings.
You'll see what I mean if you capture a sample-user system. Even if you run sysprep/generalize it gives 2 user options when signing into windows every time. I.E. It still thinks the sample-user you used to set everything up with, is a legit user.

There are ways to delete the user, but alas I haven't gone that route so I couldn't explain it as well as a few google searches about sysprep account deletion stuff could do...
There's a proper way to delete those user accounts, I just don't know it because I haven't looked.