So why is this latest RMT activity made worse by their being able to use /shout on their presumably 'free trial' accounts, and why has SE still not prevent 'free trial' accounts from spamming party invites.

No, I don't believe there are 'stolen' accounts, they appear far, far too frequently for them to be those, they have far more value to RMT to sell the accounts or use as farming bots.