Say I have a pawn in the safety zone with 2 spaces to go. It is the only pawn left to move. All the other pawns are already home. If I draw a move 7 split between pawns card, is it legal to move my pawn only 2? It's like I just split the move between 1 pawn and no other pawn.
No, you cannot use the 7 card if you lack a man (or pair of men) that can move a total of 7 spaces.
In the scenario you provided, you have one man that can move 2 spaces, and all three other men can move 0 spaces (whether this is as a result of being on Start or being in Home is actually irrelevant, as the only three cards that can move a man out of Start are 1, 2, and SORRY!). Since you cannot split the 7 card with any of the three men that can move 0 spaces, your only other option would be to move the remaining man 7 spaces, which also cannot be done. Therefore, you forfeit that movement (and thus, the turn).
It's worth noting that the only 4 cards that could be used in the exact scenario you provided (three men in home, last man 2 away) are a 1, a 2, a 4, and a 10 (forces your man one space back). Any other card would automatically cause a forfeited move because you cannot perform the movements on the card.
Interpreting the rules as presented at https://en.wikipedia.org/wiki/Sorry!_(game) :
If in this situation you drew a 10 with all your other pawns Home, you would not be able to move the remaining pawn Home. This implies that you cannot soak up extra movement on Home, and that a pawn already Home cannot absorb movement.
So I don't think you can move Home in this case.