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.
The 7 card specifically states that you can split the move up between "any" man. That means you can move your piece to and move somebody else's piece the remaining five. Considering that the other card specifically refer to "opponent's" pieces, this makes sense.