Have you tried clicking on the stealthed character's portrait and sneak attacking? That works for me when I don't see all the characters in the initiative order.
In my case, yes. The stealthed characters portrait is slightly faded out and clicking on it doesn't do anything. In some combat encounters the stealthed characters join the battle at some point with no discernable trigger for it.
Yup same here, no option to do anything; the portrait is greyed out, and when I click on it, everything else is greyed out as well. No options.
