Mark - welcome to MTW!!

Your no trade could be 2 things - enemy ships blocking the trade route or no trading post in Venice check those first.

Disappearing ships - the only thing i can think of is that they are hidden by other people's ships. If they are on the list of armies click on the fleet on the list and you should get taken there

Poor princes - if you can't wait for an assasin then a one way trip into battle is the only way - just send him by his self to a well defended enemy province - he should loose and with luck he will be killed or captured - so you can refuse to ransom him.

Try those - if no joy come and ask again