I'll try to answer to this part:

In mid-1944 would Stalin have preferred that there be no invasion?

Here is the text of Stalin's message to Churchill on June 6th 1944:

"Your communication on the successful launching of “Over- lord” has reached me. It is a source of joy to us all and of hope for further successes. The summer offensive of the Soviet troops, to be launched in keeping with the agreement reached at the Tehran Conference, will begin in mid-June in one of the vital sectors of the front. The general offensive will develop by stages, through consecutive engagement of the armies in offensive operations. Between late June and the end of July the operations will turn into a general offensive of the Soviet troops. I shall not fail to keep you posted about the course of the operations."

And then from June 11th:

"2. I have also received your message of June 10. Thank you for the information. It appears that the landing, planned on a tremendous scale, has been crowned with success. I and my colleagues cannot but recognise that this is an enterprise un- precedented in military history as to scale, breadth of conception and masterly execution. As is known, Napoleon’s plan for cross- ing the Channel failed disgracefully. Hitler the hysteric, who for two years had boasted that he would cross the Channel, did not venture even to make an attempt to carry out his threat. None but our Allies have been able to fulfil with flying colours the grand plan for crossing the Channel. History will record this as a feat of the highest order."

According to the tone of the two telegrams, Stalin was very enthusiastic about operation Overlord.

If this gets answered then I have a question that person might know as well, I heard that towards the end the British or Commonwealth forces were the only ones racing across Germany to deny as much of it as they could to the Soviets, any truth in that?