How do you efficiently use link/toon link's grab air effectively? It always deals minor damage for me (like 4-6 tops)
you can use grab airs to grab ledges
I think you use the air grab to catch thrown items, which could be very handy
You can't, but airdodges (or just hitting the standard attack button) will catch items.
Stat differences are huge. Compare Mario to Dr. Mario. You can't even really use the same play style between them.
It really isn't an apt comparison to say Dark Pit and Pit are like Dr. Mario and Mario.
We can compare the changes between each set of characters.

Gee, what a difference in playstyle! That's like... three whole moves changed! Or four, if you count the final smash. Now what about Dr. Mario?

Right off the bat, that list is much longer. Many moves have different stats. Just from the first point, up and forward tilts, up smash, back and forward aerials, and all throws. That's already a difference in nine moves, and there's more moves changed than listed there.
The move differences are also more drastic than Dark Pit's. A lot more moves differ in damage and often knockback trajectory. Forward air isn't a spike but is a powerful launcher. Forward smash deals more damage (and I believe it has less range, which doesn't seem to be mentioned here). Dr. Mario's projectile functions differently than Mario's. Dr. Mario's up special is more powerful but doesn't go as high. Plus, two of his moves are completely different (down aerial and down special). Dr. Mario is also slower and jumps lower than Mario.
Dark Pit has the same everything. The only differences are in his bow, side special, and forward tilt. They're minor, and don't add enough to differentiate the character in the same way as Mario and Dr. Mario.
Really, I'm not against clones, but claiming Dark Pit isn't just a Pit recolor is pretty silly to me.