This isn't a simpler scheme than what you propose, but some of the details hashed out. One thing you can use is a commit-reveal scheme in a two-step process (we can improve this later with state channels or batching zero-knowledge proofs, as you suggested).
You can't use the IPFS hash modulo the sender's address as the commitment, b/c the front-runner will ...
The following Ethereum web wallets are now seeing increasing use alongside Metamask for various dapps, none of which require installing a chrome extension.
Democracy <-- the last one is my project
For a comparison of why you might use one wallet over another, and general design considerations that go into making a web wallet, I'...