Can't Login to SharePoint as an invited External User / Can't Remove All Traces of External User with Microsoft Account

Question

Hi TechNet,
I have an MS SharePoint Online (SharePoint Plan 2) team site, quite simple, one document library etc.
I have successfully added all users (E3) within the organisation to groups, and permissioned correctly.
I have added myself (separate organisation, also Office 365 E3) as an External User, and have access to the website without any problems by authenticating with my Organization account e-mail address.
I have a single user (separate organisation, also Office 365 E3), who's setup is identical to mine (Also Office 365 E3).
However, when this user is added as an External User, they are unable to login, and get "Sign In is not complete":
That didn't work
We're sorry, but [email protected] can't be found in the CLIENT1.sharepoint.com directory. Please try again later, while we try to automatically fix this for you. 
Correlation ID: dc1f7f9c-092b-20b8-7b35-89348ba22f71
Date and Time: 3/20/2014 7:06:55 AM
URL: https://CLIENT1.sharepoint.com/
User: [email protected]
Issue Type: Partner User Invalid.    
I then remove the user using the Site Collection, and using the PRofile Manager, and using Remove-SPOUser, and using Remove-SPOExternalUser. Which is great, he's gone. However when I go to add him back to a group, as soon as I type his e-mail address, it
'Resolves' into his full name! If I have completely(?) removed him form the site, how is he being resolved? And therefore me trying to remove him to re-add him to try and solve the user/directory/auth issue is not working.
Furthermore, upon clicking on said client's username inside SharePoint (after I've 'added him back' of course), his ID, in format: i:0#.f|membership|live.com#[email protected] has an entirely different e-mail address, his Microsoft Account! 
I'm assuming he must have been already signed into his Microsoft Account when he clicked on the External User e-mail invite? If so, I clearly do not want this, how can I remove lal traces of his Microsoft Account, given that I have gone to the lengths as
detailed above?
I have already completed these steps: http://community.office365.com/en-us/forums/148/p/228263/709905.aspx
Some possible further reading regarding Microsoft ID's and Organization ID's:
http://sergeluca.wordpress.com/2013/09/23/sharepoint-online-and-external-users-this-invitation-has-already-been-accepted-with-another-account-bug-or-feature/
Please let me know if you need any more information regarding this issue, and thanks in advance to anyone who can shed some light on this situation for me and anyone whom encounters it in the future.
Regards,
Evanly.

Answer 1

Hi Scott,
Thank you so much for taking the time to read and respond to my issue.
Certainly, it makes sense that regardless of where the invitation it sent, the user would authenticate with their Microsoft ID.
In my case, I want the user to authenticate using their Microsoft Organisation ID, that they use for their seperate Office 365 account.
This is the way I was able to log in, and worked great. With my client, they are unable to access Sharepoint because once they sign in with their Microsoft Organisation / Office 365 ID, they are told they are not in the directory, because their Microsoft
ID is in the directory and it doesn't match up.
I am simultaneously trying to 1) Remove all traces of this users Microsoft ID, which so far using the above steps, has been unsuccessful; and 2) Invite the user using his Microsoft Organisation ID, and have him authenticate with that (which is proved to
work, as my account uses this).
Looking forward to any more suggestions. Thanks in advance!