since msn and plus! seem to work for you with another .net passport the only thing i can ask (sort of a stupid question really) is .. when uninstalling plus! did you uninstall your preferences as well?
perhaps you left them behind, and each time you sign in with your main account, the problem you had in the beginning with the "custom name" for the mobile only contact, is causing the problem.
... another thing you can try is maybe (when you sign in to msn without plus! running) delete your mobile only contact from your contact list, and sign out. than sign back in again but with plus! and see if that works out for you. (if it does, than add your contact again.)
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