Definately a tough call. I tend to agree with option A to continue any of his unfinished stories. As you post new chapters, go ahead and post them as Marco Polo but with a disclaimer at the beginning of each chapter noting that you are "Capt'n Crunch", Marco's brother, and finishing his stories. Story announcements under Marco's name would be ok too, but if you do comment on any of the threads, it would be best to do it under your name. In other words, try to limit the use of his account to just story posting and announcements, and even there, please use the disclaimer.
Usually we don't like (nor allow) use of someone else's account - but in a case such as this where the original account owner has unfortunately passed away, I think we all understand the desire to leave a tribute of someone close to him finishing his stories.
If you have any questions, needs, etc - let me know.
Trebs
[PS - for those that don't know, red text indicates a statement made from the role of an Admin]