I'm going to have to go with not changing the clinch rules at all. Combat in Exalted is deadly. A lot of times I have seen characters die in one hit, because they used a charm and then couldn't use a perfect defense.
Now, there are a couple of things you can do to mitigate the problems you are having with a single opponent getting grappled and then killed. One of the solutions involves the hearthstone that makes you totally immune to grapples (It's called the Freedom Stone and is in the 1st book). Another is looking at what kind of DVs your bad guy has. Hero A still has to get over the DV in order to grab the baddie, and there are plenty of ways you can make that higher while the attack is going on (or use a perfect, which your Celestial level opponent should probably have).
For Dragon Blooded, remember that they have reflexive Excellencies, that do not count as a charm use, and they do actually have "perfects" but they're really weak (the Parry only works against non-charm supplemented attacks, the dodge is a bit better). They also have Anima effects that cause damage. A fun proposal would be a Fire-Aspect gets grappled, but he has a hearthstone which increases the damage of his anima effect by 4, and he already adds his essence into that, and suddenly he's doing 7L environmental damage that can only be soaked with Stamina...and someone is holding on to him! (Also fun is the Mask of Winters does 1L per tick to everything within 10 yards automatically and has a scene long perfect dodge. :D)
Some other things to remember with big bads that don't have perfects. Spirits have a really good extra action charm, that can generally be used to pummel a single opponent into the ground, which you could use on the martial artist. They can also dematerialize.
I'm rambling now, but I think the point is that there are many ways you can counter a grapple without cheeseballing things, you just have to get a little creative with it.
And finally, Exalted is an epic game. The players are epic and they fight epic things, so don't be afraid to give your baddies some nifty artifacts (Perfected Kata Bracers are super nice for Martial Artists, btw).