If a good order SMC fails a MC, he breaks and gets a DM counter
and has to roll to see if he is wounded or killed right?
No. He is only wounded on an IFT result of K/# where he is selected as the unit that suffers a casualty reduction, or as stated, he is already broken and fails a MC, etc. He immediately takes a wound severity die roll. He might be KIA (5-6) or he might just suffer light wounds(1-4). If he suffers light wounds he gets a wounded marker which gives him a +1 to any future wound severity die roll.
If he then fails another MC after already being wounded/ reduced, he would then roll another die with a +1 modifier to see if he is KIA (but with a chance to suvive this die roll and live to see more combat?)
This is not quite right because you are combining/mixing the effects of being wounded and having broken morale. A leader can be wounded but not broken. For instance the leader is selected to suffer the CR from a K/# IFT result but then passes his morale check (even though, remember, his morale and leadership modifier have been reduced by 1). Or a wounded leader breaks, but then rallies. If a wounded unbroken leader breaks he does not make a wound severity die roll. A wounded broken leader that fails an MC would take a new wound severity die roll, as would a wounded leader unlucky enough to be chosen to suffer the CR from a second K/# IFT result, or a wounded broken leader that rolls a 12 on a subsequent rally attempt, or an unbroken leader that takes a MC and rolls a natural 12. Any subsequent WS dr is 50/50 (1-3 lives, 4-6 dies)