TWAIN and WIA are terrible and slow, so first I will do the Workflow Scanning, then move on to TWAIN/WIA
Make sure the firmware is up to date, currently that would mean 061.121.226.12400.
Make sure the time is right, it matters with SMB2-3 and it matters if you are on HTTPs.
Port isn't 139 any more, it's port 445 for SMB2 or SMB3
Make sure the username is formatted as Windows requires, it isn't just usernames any more, there is Domains, HOSTS or an email address in wWindows 8 and above. Add the user in the folder permissions and security, get the proper format in the brackets.
Don't put the folder inside of the Users folder (C:\Users) Use the root of a drive and send a shortcut to the desktop if need be.
Right-clicking the folder and choosing Share With only applies to Windows Homegroup, no printer will access that.
So I shared the Xerox (C:\Xerox) folder on my Windows 8.1 test PC and verified the settings as above, scans on the latest firmware with no issue using the following template.
As for the TWAIN/WIA issues. This is the driver I used.
Installed it manually via Scanners and cameras and chose the driver via Have Disk (Choose WIA, not TWAIN)
After it completed, selected it and clicked Properties > Administrator Options and connected the driver to the printer via the IP address
Then in Windows Fax and Scan choose New Scan and select the printer just installed by name.
Try with Windows Firewall disabled if it still wont work
If you go to the 7556 does the WIA template show up?
If yes, does it work if you try it? (You will need to put in the user/pass when initiating from the MFP)