A little more on your Q.4
Not only can you continue to use MQ Explorer, but you can continue to use any client connected MQ administration tool. In essence, to use a tool with IBM MQ on IBM Cloud, you need to be able use a client connection, and to specify a user ID and password on the connect call.
I mention this because we have a few :-)
Please see Use MQGem tools with IBM MQ on IBM Cloud