Tyber Zann vs G0-T0 will be the Round 3 of the Heats. And you make very good points, Grievous holds all the guards. However all Mandalore has to do to win is kill Grievous... which at least if he managed to get a 1v1 engagement is definitely possible.
But the only way Mandalore would get a 1v1 engagement is by destroying Grievous's army. Which won't happen.
I think this will end in one of three ways. All favor for Grievous.
1.) The war plays out and Grievous wins. He finds Mandalore and just kills him with his army and/or blowing up where ever he is.
2.) Mandalore is killed by an assassin. This is a bit of a strech, but it's needs to be said that Mandalorians aren't the kind for hiding and playing under-handed like assassins. The CIS, on the other hand, is. There will at least be attempts.
3.) 1v >2 I think it goes without saying, from what we know of Grievous, that he will not go 1v1. He would only do that as a last ditch attmpt. But he's not going to need to take those drastic measures. He will, instead, save Ventress to deal with Mandalore. Grievous + Ventress + Magma guards. Or at least Grievous's MagmaGuards. He always has those things with him.
Mandalore is a great warrior, and he's proven himself in combat. Unfortunately, he has only ever been challenged by single opponents. Overwhelming enemies, the Force, and his sense of honor would doom Maqndalore in a fight against Grievous and his guards.
I'm curious though. If they did go 1v1, what gives you the impression that Mandalore would win?
We are talking about the same Grievous, right?