The thing is, as you said, Grievous is a coward and won't get captured. We've seen him escape many times, he'll do it again. In this Kaggath it would not come down to a one on one fight.
The only reason why Grievous escapes so often is because of Plot Protection. If he didn't appear in Ep3, they'd have killed him off already. That's all it is. For such things like Vs debates and so forth, that won't cut it. He doesn't have the Plot keeping him from being offed. The same can be said for Malgus. He may not get to stay as the "Emperor". Here, Grievous won't have to wait for Obi-Wan to smoke him. And has been said before, Malgus has other Sith on his side, including Darth Serevin.