Recently, my work allowed us to connect to Exchange via Mac. I have used, for years BusyMac's BusyCal because it has far superior Alert window options (that just matters a lot to me.) I put my account creds into BusyCal and have been getting sync perfectly. Today, not sure why, I wondered what would happen if I accepted a meeting from BusyCal instead of Outlook.
Answer: It keeps the invitation in the Inbox! (Something I have wanted so much, too).
Up to you if having that matters enough to buy BusyCal. But at least we have a solution until MS build it into Outlook for Mac.