[SalesForce] REST API & Validation Rules error

Short Answer

You need to turn off immediate and adjust or disable your validation rules.

Explanation

Both issues you're having are related to the immediate=true parameter on the action function. This parameter forces the action function to run immediately without running (1) any getters/setters, and (2) any front-end validations.

For (1), since the getters and setters aren't run, none of the field changes make it back to your controller. Hence the record you're saving doesn't include any of the new input.

For (2), you're only bypassing front-end validations, specifically fields you've marked as required in the visualforce page. Validation rules are ALWAYS run no matter what, you can't turn them off. You'll need to disable them or adjust them so they don't apply to new records.

You do need to use the Tooling API to update Apex classes. Updating an Apex class is a more complex operation than creating a class, since you may have a set of updates that need to be applied as a single set of changes, and it's an asynchronous operation.

The Tooling API docs have some sample code for REST and for SOAP. I built a very basic code editor as an example in Java that pulls it together - here is the code to update an Apex Class. I've added some comments to show the flow:

try {
    // The container for the entire request
    JSONObject metadataContainerRequest = new JSONObject();
    metadataContainerRequest.put("Name", "SaveClass" + id);
    JSONObject metadataContainerResponse = ToolingApi.post("sobjects/MetadataContainer", metadataContainerRequest);
    System.out.println("MetadataContainer id: " + metadataContainerResponse.get("id"));

    // Wrapper for changes to a single Apex class - you could add several of these
    // to a single metadata container
    JSONObject apexClassMemberRequest = new JSONObject();
    apexClassMemberRequest.put("MetadataContainerId", metadataContainerResponse.get("id"));
    apexClassMemberRequest.put("ContentEntityId", id);
    apexClassMemberRequest.put("Body", body);
    JSONObject apexClassMemberResponse = ToolingApi.post("sobjects/ApexClassMember", apexClassMemberRequest);
    System.out.println("ApexClassMember id: " + apexClassMemberResponse.get("id"));

    // Apply the changes
    JSONObject containerAsyncRequest = new JSONObject();
    containerAsyncRequest.put("MetadataContainerId", metadataContainerResponse.get("id"));
    containerAsyncRequest.put("isCheckOnly", false);
    JSONObject containerAsyncResponse = ToolingApi.post("sobjects/ContainerAsyncRequest", containerAsyncRequest);
    System.out.println("ContainerAsyncRequest id: " + containerAsyncResponse.get("id"));

    // See if the update is complete, keep checking until it is
    JSONObject result = ToolingApi.get("sobjects/ContainerAsyncRequest/"
        + containerAsyncResponse.get("id"));
    String state = (String) result.get("State");
    System.out.println("State: " + state);
    int wait = 1;
    while (state.equals("Queued")) {
        try {
            System.out.println("Sleeping for " + wait + " second(s)");
            Thread.sleep(wait * 1000);
        } catch (InterruptedException ex) {
            Thread.currentThread().interrupt();
        }

        wait *= 2;

        result = ToolingApi.get("sobjects/ContainerAsyncRequest/" + containerAsyncResponse.get("id"));
        state = (String) result.get("State");
        System.out.println("State: " + state);
    }

    // Housekeeping - delete the container
    ToolingApi.delete("sobjects/MetadataContainer/" + metadataContainerResponse.get("id"));

    if (state.equals("Completed")) {
        // Success!
        return "redirect:../classes";
    } else {
        // Failure - show errors to user
        map.put("body", body);
        map.put("errorMsg", result.get("ErrorMsg"));
        String compilerErrors = (String) result.get("CompilerErrors");
        if (compilerErrors != null) {
            JSONArray parsedErrors = (JSONArray) JSONValue.parse(compilerErrors);
            map.put("compilerErrors", parsedErrors);
        }
        return "classDetail";
    }
} catch (RuntimeException e) {
    map.put("errorMsg", e.getMessage()); // TODO: better looking error
    return "classDetail";
}