1) I use my centre backs on zonal, I did use man-marking before, but when the opposition is good it fails, especially when your players are slow. I tend to go with the low closing down non tight zonal aproach when using slow defenders against fast strikers, basicly my idea is to put them between the goal and the striker. I also go with light tackling sometimes to limit them rushing the oponent. Going one zonal and one manmark would seem suicide in my book since both players would be able to end up on the same guy leaving a huge gap in your defence.
Do your DC's have good positioning and anticipation? a fast defender might be able to cope without it, but a slow one will be as bad as Thuram is right now
2) In my book, to get interceptions you have to press, otherwise the poor passes that lead to interceptions will drop. So you'll need the players infront of him to press and him being a bit stand off. Try keeping his pressing slightly below average and having the players infront to have higher pressing, If you're using 2 dm's I would chose the one with higher anticipation and positioning to do the interceptions and use the other to press, using both your dm's in a stand off approach might give the opposition too much time on the ball as you only have one mc/amc to press with in the centre of the pitch.