I would highly recommend James Felici's The Complete Manual of Typography. Goes through all the core concepts of typography and why they matter, and provides a number of guidelines.
> It has fallback to SMS functionality, but SMS apps are rejected from the Apple App store.
Which guideline are you thinking of? My search for "SMS" only found two unrelated guidelines:
> 1.1.6 False information and features, including inaccurate device data or trick/joke functionality, such as fake location trackers. Stating that the app is “for entertainment purposes” won’t overcome this guideline. Apps that enable anonymous or prank phone calls or SMS/MMS messaging will be rejected.
> 3.2.2 (iv) Collecting funds within the app for charities and fundraisers. Apps that seek to raise money for such causes must be free on the App Store and may only collect funds outside of the app, such as via Safari or SMS.
The purpose of the signature is to establish legal solemnity, not identity or authority. It's why you can sign with just an X, and why really important contracts still need to notarized (which does establish identity).