It's not impossible to do. From the POV of an audience in 1970/71 with no knowledge of TMP to come, Kirk becoming the youngest Admiral in Starfleet history would be seen as a well-earned hero's reward. Bonus points if he gets a new girl (Admiral Lori Ciana?) in the bargain. With Spock, all they have to say is that he's returning home and going on a retreat (vision quest?) to find inner peace - something hippies of the time were all about. No need to go into detail on exactly how he's going to find inner peace until the reveal at the start of TMP.
McCoy's arc is going to be the trickiest one to do, especially if (as I earlier surmised) a dramatic fallout between him and Kirk happens. Another contemporary parallel - the Vietnam soldier returning home, nursing emotional scars, trying to pick up the pieces of a long lost life. No way in hell Gene could resist that. Since Joanna McCoy hasn't debuted in any version of Star Trek to this point, this would be the perfect spot to introduce her as Bones tries to reconnect to something, anything. Leaving it with him, and the impression he's going to be all right as he returns to being a good 'ol country doctor on Earth, would provide the needed 'happy ending.'
McCoy's arc is going to be the trickiest one to do, especially if (as I earlier surmised) a dramatic fallout between him and Kirk happens. Another contemporary parallel - the Vietnam soldier returning home, nursing emotional scars, trying to pick up the pieces of a long lost life. No way in hell Gene could resist that. Since Joanna McCoy hasn't debuted in any version of Star Trek to this point, this would be the perfect spot to introduce her as Bones tries to reconnect to something, anything. Leaving it with him, and the impression he's going to be all right as he returns to being a good 'ol country doctor on Earth, would provide the needed 'happy ending.'