I could be wrong, but I believe you need a signed contract before you can apply for a work permit? It sounds like it's taken a couple of days to secure that for Yoshida and Mayuka? Things have gone quite for a couple of days... Just sayin'.
From the FA guidelines;
I'd assume that if a player was selected for an international youth squad, then he wouldn't be considered available for selection for the 'A' squad?
The '75% thing' is a UK Border Agency rule. I don't believe it's enforced in any other countries?
http://www.thefa.com/TheFA/RulesandRegulations/FARegulations/NewsAndFeatures/2009/work_permits_players.aspx