You may be giving yourself unnecessary constraints that would make it harder to achieve the result. You need a structural layer that will stay where you put it during creation, and a finish layer that will bond with it and flow enough to self-level into a smooth surface. The structural layer needs to cure into the right characteristics for strength and not be brittle. The finish layer needs to cure with different characteristics to achieve a different purpose. Trying to achieve the two different purposes by just adjusting the viscosity of the same epoxy isn't a good way to do it.
Creation stage: Anything that's just a viscous liquid will flow. That's a problem when you're working against gravity with non-horizontal surfaces; even horizontal surfaces if you don't constrain the edges.
There are a couple of ways to overcome that. One is to use a material designed to not flow. There are epoxy putties and clays that are designed to be worked like clay. You can form it into what you want and it will stay like that. So you could press it against your form and work it into a flat surface of the desired thickness, or roll it into a flat sheet of the desired thickness, lay it onto the form and trim the excess. That might work well with your idea of making sections and then joining them.
Another is epoxy paste, like Bondo, designed for purposes like repairing car bodies. It can be spread and shaped and will stay as you put it. You could just spread a layer of the desired thickness on your form.
A different way to avoid slumping is to work in layers, impregnating liquid epoxy into something that holds the epoxy in place. You can use something like nylon or fiberglass mesh (similar to what's used to strengthen repairs of cracks in drywall or concrete), or a fabric. You make each layer not much thicker than the mesh or fabric, so there isn't free epoxy to flow.
Strength: Epoxy tends to be brittle. Some epoxy clay or paste can be even more so; they contain fillers that may be optimized for purposes other than strength (there are also clays and pastes that contain reinforcing material, so it's stronger than plain resin). If you were to cast a thick piece, it would have plenty of strength. But creating a relatively thin sheet for the shell won't. One way to strengthen it is to reinforce it.
If you use epoxy clay, you can embed mesh into it, or create two thinner layers and sandwich the mesh between them. If you use layers of liquid resin with mesh or a non-stretchable fabric like fiberglass, that provides the reinforcement. A relatively thin skin of resin and fiberglass is strong enough for purposes like car and boat bodies. If you use a product that contains reinforcing fibers (like certain versions of Bondo), you wouldn't need additional reinforcement; just figure out how thick the shell needs to be for the required strength and rigidity.
The structural epoxy shell doesn't necessarily need a finish layer. You can think of the whole shell as a thick finish; it just doesn't have a super-smooth surface. If you color the epoxy used for the shell, you could just smooth and polish the surface. The only reasons to add a finish layer are for the color or to save some of the work of polishing the surface.
If you make the shell in sections and join them, you can polish each section before you join them. If you apply a finish layer, do that to the entire unit after the sections are joined. You would likely use epoxy clay or paste in the joints, and it's better to bond that with the shell material than to a finish layer.
It isn't really a problem getting finishes to bond with epoxy. You can choose a finish material that's the best for the job. There isn't a good reason to use a low-viscosity epoxy for that. You can pour resin over a flat surface and get a nice finish, but that won't work on non-horizontal surfaces; it will flow.
To finish a non-horizontal surface, you need a material that acts like paint. You need to be able to apply it in thin layers that flow enough to blend and self-level locally into a smooth surface, and quickly harden enough to not flow more. Epoxy resin is terrible for that purpose; it has all the wrong characteristics. You can do it in an industrial setting, but it isn't practical otherwise.
There isn't a need to reinvent the wheel. Applying a tough, nice finish to epoxy has already been perfected, is done routinely, and is readily available. It's the finishing step of car body repair. You can get automotive paint in any color, and clear coat, in handy spray cans. That will do a much better job, more simply, than trying to apply epoxy resin uniformly to big 3D pieces.