will all my custom Wordpress themes and plugins run on EmDash?

For plugins, no.

1- EmDash plugins are written in TypeScript, not PHP

2- EmDash plugins have a specific permissions model, where they need to explicitly request access to certain things.

3- WordPress plugins just invoke things. EmDash plugins have a defined API you use to talk to different capabilitites

4- Those capabilities are totally different, and at a different abstraction, than what WordPress provides.

Beyond the look of the admin interface and publishing flow, I don't see how this is a "Spirtual Successor" to WordPress at all. Its a CMS, designed from scratch, for a serverless world, using CF proprietary capabilities (D1 Databases, R2 for image/media storage, their workers for running things).