Redwhait HP, do you mean leveling Cuthberts with all your might? oh dear. All means are valid for victory, maybe a diplomat can also get trash and joiners at 13, but is it worth counting on that??

Explain why.
IMHO this is only if the skill difference is around 5 levels and the diplomat has bad luck. Or an early Cloak of the Undead King, of course.

This is just chaos, there can be all sorts of scenarios, and the Cloak of the Undead King has nothing to do with it. Even just a regular Necro without the Cloak on XL can out-pacify, while the diplomat will be busy joining all sorts of junk.
+ various scrolls and a 4 Dimension Door and it's game over for the diplomat, or relics behind shifted guards, etc., there are actually many scenarios.
Well, leveling Cuthberts isn't mandatory, but everyone knows that clerics have the highest chance for Diplomacy. And if there are no suitable options, then you can certainly make do with them.