Is there some beta stuff out I can test with? Or documentation?

GraphQL API has a lot of useful documetation on reading data https://us-01.infra.3speak.tv/v1/graphql
As well as...
https://github.com/3speaknetwork/spk-graph-client
https://github.com/3speaknetwork/spk-indexer-node

What will be onchain and what can be stored local or on 3speak network?

All offchain content will not stored on chain. There might be references on chain for indexing purposes but more TBD on this area. (specifically the light account side of the project)

Email login is very much possible. However, it requires a service that handles the email part and storing the private key of the user. Already a fair bit of services exist to this for Ethereum/others.

i mean something like: @did:key:app.username to make it super easy. Mostly to allow other websites to add token balances without a huge education program.

I'd recommend having an internal resolving within the platform that resolves an @username to did:key:etc. The did:key only accepts a public key as a value. Anything custom will need to be specified another way. We can also resolve the did:key via caip10links. Meaning you can go to @0x01234567890 and get a profile.