JVM can run on any platform that supports Java, as you’re building Kotlin directly into Java bytecode.

Multiplatform is for building native applications while using a single backend logic. You’ll have to write separate handlers for everything unique to the platform, according to documentation.

It’s pretty easy to just not put the AI tag on things, or to strip such things away from an image.

The point of a digital signature is to announce that you made this document, as it exists at the time of writing. Once a change is made it should no longer identify as signed.

Most institutions don’t use this functionality, despite the usefulness of it. At present, I’d recommend using it for publicly distributed files to protect against bad actors publishing a document that pretends to be yours.

As for legally binding, ask a lawyer. Generally, things are legally binding if they’re signed by all parties. The specifics get funky, but a digital signature is a solid step for announcing that you did this thing at this datetime and a judge should recognize that if it comes down to it. Bonus points if all parties attach their digital signatures.