Can you build a app with Razor pages and use Blazor components with render-mode=”WebAssembly”?

I haven't personally used. I think you can, but I heard the experience is not very pleasant.

Blazor WebAssembly so that I can use C# rather than JavaScript

But Blazor isn't just WASM. It has Server-Side mode as well. If you write a component using C#, it will work on WASM and on Server-Side.

Razor is better for developing apps when SEO is a priority

You can use Blazor Server-Side, and the SEO will be just as good. If you really want to use WASM, you can mix modes by starting with Server-Side rendering and then switching to WASM. There’s also pre-rendering support to help with SEO
There are tons of tips and tricks like this article. If you want a fully WASM-based solution, you can use the BlazorWasmPreRender.Build package. There are many ways to build an SEO-friendly Blazor app.

so that I can use C# rather than JavaScript

Blazor doesn't replace JavaScript. If you want to do something advanced, you'll still need to use it. If you need DOM manipulation, or browser APIs (Local storage etc), you'll end up writing JavaScript code and invoking it through IJSRuntime.