For flying I STRONGLY suggest skipping the swept blade props. By all means make some "scale props" for static display. But stick to commercial props for actually flying. As you say you don't have enough data to fully engineer a properly safe prop. And you can't just toss carbon fiber at this and trust that it'll hold. Carbon fails too when the forces get powerful enough.
800 square inches of area is a lot of wing. And a model that size at 10lbs is not out of the question at all. But you WILL need a lot of power for a model of that size. Power that will use bigger props than 9 inch. For performance that matches the design role of this sort of fake scale design you'll want around 100W/lb of model. So 1000w of motor power.
Will you be running actual contra-rotating props or fake it by using two props on one shaft both turning the same way? If the first then both can have the same pitch or the second can be one pitch amount less because of how it meets the air off the first prop. If you fake it and go with two props on the one shaft then the rear prop should be the same or slightly greater pitch. And you might well find that this combo works best when it's not quite at 90° so the front prop acts as a prop and the rear acts sort of like a booster flap for the second one. But if in doubt just put them at 90 and run the same pitch for both.
.75oz cloth is the super light stuff that is used over balsa as a prep finish for painting. It is not very strong. If you're going to use epoxy and fiberglass over foam you will want to use cloth layups similar to that used on vacuum bagged large glider wings. And I believe that this is more often 2oz cloth with second patches at higher stress points.
Making a plug and then a proper female mold for such a plane would be a major amount of work if you are not planning on making and selling at least a few copies. If you're after a one off a far easier way is to make up a skeleton of vertical and horizontal keels and formers and then insert blocks of Styrofoam in the openings, carve and sand it all to the right shape and then glass over the foam and framing with epoxy and glass. Also this way you can add in hard points for landing gear and wing mounts. The foam in this case is roughly hollowed out so you leave a generous amount of foam but at the same time leave yourself lots of room for the radio and battery and stuff. For a one off this is a lot faster and easier. And in the end if you do decide on making a female mold to make more then you can use the prototype as your plug for making the mold.
It's certainly a funky looking design. Good luck with it if you go ahead and I hope my suggestions help out.