Well I think varying sett sizes in a single outfit look smashing so I wouldn't worry about that as long as the waistcoat is cut on the bias.

I would go with the shaped bottom and no lapels.

A satin back is good if you want it to be cooler. I am only making a full back vest for my tweed ensemble because I intend to wear it when it's colder.

Here are some pictures that show how sett sizes compare when worn together.