I use rum in Porter a lot. I call it Pirates Porter. I didn't know anyone else had used that name.
As to the MM. Add the MM at bottling. That way you will retain all it's character in the bottle. You can then complete the batch as normal, and when it comes to bottling, just rack half to the bottling bucket with the MM and priming sugar, bottle that, and then rack the rest into the bucket with the rest of the priming sugar and the 'cheap stuff'.
I see no reason to use MM. Just use a bourbon you like. If you like it straight, you will like it in the beer.
Always add any liquor at bottling. There is no reason to add it earlier (unless you used it to soak some oak or spices in first.