Question: Why is it not possible to refer to exported types in imports?

The Component Model does not currently allow you to refer to exported types in imports, which feels asymmetric given that one can easily refer to imported types in exports. This becomes a practical concern when attempting to virtualize interfaces such as wasi:http/incoming-handler, which contains a handle function with parameters which are handles to resources which the guest is importing. The most natural way to virtualize that interface is unfortunately impossible since it would require importing a function with parameters which are handles to resources which the virtualizing component is exporting.

Is there a fundamental reason why this is not possible, e.g. by allowing a component to declare a type, refer to it in an import, and later export it?

Start with the Component Model specification rules for imports, exports, and type declarations, then trace the wasi:http/incoming-handler interface and its handle signature. Done would require a documented design decision explaining whether exported types can be referenced by imports and, if so, the required model changes.