(1) not strictly json validation, but general validation issue - how zabbix sender data is validated.
a) header is optional - skipping it works just fine.
b) looks like sender itself is not sending the header.
c) if header is included, length is ignored unless it's 0 - then sending silently fails.

hello, we use this code in item validation :

try {
JSON.parse(value);
    } catch (e) {
        return e;
    }
    return value;
}

with a trigger find(itemkey,#1,"like","SyntaxError") = 1

or just

return JSON.parse(value);

with a nodata trigger.
But a "predefined" validation of json can be a best approch.