I'm the GM, so I thought I'd ask before it hits during play. I think the prerequisites are similar to subdual or ambush, the target can't know you're there (or suspect there's an attack coming) and it's harder if the target is in melee.
One attraction of using subdual is that it's a yes-no answer. You succeeded, or you didn't. The downside is the idea that you'd need to train in "subdual: empty-handed" and it may prove to be a very rare skill to use.
With grapple, what result do you consider successful enough to say "you have your hand over their mouth"? Any crit? Any crit that gives a penalty? Any crit that stuns? Grapple seems a plausible skill but it brings more questions. It does, however, mean the character has a skill with benefits beyond just the hand-on-mouth option.

It's based off of what your GM would rule.

Grappling might work because your grabbing your target from behind.  But subdual can also work because your looking to cause your target to lose consciousness.  Ambush might also work, too.

The grab-from-behind, hand over mouth attack is pretty standard/common in movies, books, etc. Add a rag with chloroform and it's more effective. What skill would you use for this, in RM? I can see a case for subdual or grappling. Others?

There's been a lot of movement with harp the past few months - Bestiary, Subterfuge, Garden of Rain, Beyond the Veil ... so will Cyradon be happening as well soon?

My take on the magent spell is that it does not have a fixed duration because it covers the assassination attempt and not a specific time period. The actual masked time period is determined by how long time is needed to satisfy the shadow/unseen/false assassin effect. The balancing factor is that the spell is strictly limited to assassinations so you cannot use it to for instance steal a book...unless you manage to also perform an assassination while you steal the book.

Where have you seen that the spell effects are limited to assassinations ? The spell text certainly contains no such limitation, it just says "all deeds".

Add the 2H mods to the initial mods and that is your result.
No change to thrown bonus since you cannot throw with both hands effectively, heh.