1: A normal seat tube is quite thin at the top because it is supported by the top tube and the stays near the load point so there is little bending moment supported by the tube itself. If you move the load point away from the tube junction, the tube needs to be reinforced to carry the extra bending moment which increases as the square of the distance, so doubling the tube extension quadruples the bending moment. Whoever builds the frame needs to be able to calculate the extra tubing thickness required to support this extra bending moment.

A thought experiment: place the frame with the seat tube horizontal and the head tube up, supported at the dropouts and the seat stay junction. Now jump on the end of the seat tube extension and see what happens.

If you think the frame will never see that much load when ridden, think again.

2. Darren McCulloch (Llewellyn Bikes in QLD, Oz) designs very nice lugs for sloped geometries, available direct and from Ceeway in the UK and Nova in the USA.

3. In theory probably not, but in practice probably yes. A professional will make your frame his way, which will probably be better than your frame your way.