I've been screwing around with your advice and I've come up with a pretty solid combo.
Grab, Dthrow
Grab again, nair. marth can nair out of this, at least the cpu has. Human opponents have not.
grab once more, dthrow.
Knee. If you are quick, he can't tech the dthrow.
If you started the combo at zero, which is when it's meant to start from, he should be at 50% from this knee. This is the perfect time to go into a double knee. If he's not off the stage, you're doing it wrong.
Finishing with proper isaiguard will make this a definite zero to death combo.
EDIT:
This is actually solid, and I think I should explain it a bit more. The closer you are to the percents that I will provide below, the more knees you can get in at the end. I've gotten three.
At the beginning, they should be at zero. Dthrow should bring them to 7, but decay will applies sometimes. It matters not. Regrab and uthrow, bringing them to about 13-12 percent. The nair should have them at 25ish. Dthrow should have them 32ish, and knee will have them at 50. 2nd knee, though not mandatory, should have them at 65ish, and then if you want to be cool and get a third knee, they should be at about 75. I haven't run the percent's attack by attack, but if I see them at any of these percents I tend to pick it up from that spot.