We can't actually use skype logs, but I doubt it matters in this case.
You mentioned yourself that you abided to the TOS stating he wouldn't be held responsible for bans. And the account wasn't just in his possession, it was in yours too.
While your theory is possible, there's no way to actually prove anything at all. We can't prove who got that account banned, and therefore there's nothing we can actually do about this.
Sorry for the loss of your account, but you said yourself it might just as well have been a delayed ban.