As for getting Elasticsearch URL - it is possible, by reading kibana config in your plugin. All you need is add server function like this:
server.route({
path: '/api/<some your url>/{attr}',
method: 'GET',
handler(req, reply) {
reply(server.config().get(req.params.attr));
}
});
and then perform get requests from you client side code in plugin.
You can get any settings from kibana.yml (look throw the file and use )))