"ooo, lets send the crew back to present day earth! never done that before....except for in Tomorrow is Yesterday...and oh yeah...Assignment Earth...oh..and...the Voyage Home...oh..and then there was futures end!"
You're confusing episodes involving a similar theme - crew goes back to 20th century Earth - with two episodes where the degree of similarity is go great as to make them virtually the same episode with different casts and small changes.
Region of space fatal to most of the crew.
Check.
One crewmember is immune.
Check.
Crew goes to sleep while the ship traverses that region.
Check.
Immune crewmember is left alone to take care of the ship.
Check.
Immune crewmember is used to large groups and is uncomfortable being alone.
Check.
Immune crewmember begins to hallucinate another person.
Check.
And so on.
Sighting Tomorrow is Yesterday as being virtually identical to The Voyage Home just because they involve the crew going back to the twentieth century seems a bit silly to say the least.