You pretty much got it down. When resining the outside for the first few layers, you should add a good amount of supports on the inside and maybe not apply resin to the whole helmet at once to prevent warping. Resin and Bondo for the inside should give you a nice smooth inside, But I'd put at least one layer of fiber glass inside before the rondo to give it more strength and prevent it from shattering too easily. You can just sand down the outside, but for smoothing some parts, bondo can be useful so you don't sand through your helmet. In the end, you just have to try and see how it goes, as a lot of things like resin, way of application and paper influence how things go.