Welcome to the hell of "No custom charms allowed."
DP did say that he'll allow eventually, but...
Anyway, for homebrew, you need to define from which Hell does the charm in question originate, and how does it connect to the themes of that hell. Connection doesn't have to be very strong, but it needs to be at least a little bit there.
Charm to spare ppl from collateral damage... I dunno. Hells we got available aren't big on mercy and care for people who do not bow to us. Charms that exclude our underlings from AoE damage and such are possible, but excluding random civvies is a bit off-theme for Lanka, Boiling Oil, Kakuri and so on.
We should experiment a bit, try to recraft it, once we increase the dice pool a little and can guarantee 20+ sux results. Would be a good general litmus test on how gross over-roll interacts with crafting tasks.
4 points of soak at difficulty 5 is absolutely insufficient to render ppl with guns irrelevant. We tested that on Molly's hide. Shintai form Molly is generally invulnerable to ppl with hand-held firearms, though.
We have BSM, which has ease of activation baked in. DP confirmed that we are able to intentionally trigger CCC in locked room scenarios. Shintai Signature is another, and so is Barren Waste Infliction. That's -6 total from core charms, and we can get another -5/-7ish~ from Linear paths through Alchemy, Fortune, merits and other paths.
TLF does not exist in a vacuum; I can see a watsonian argument for allowing it to easily trigger for crafts and prep, but Doylist argument of "There is something like -12 total difficulty adjustment available already, hold your horses" feels very strong to me.
Also, we can craft stuff that'll give us more soak. IDK if we can do the same to boost Perception.
Alchemy/Enchanting 3 allows us to craft an artifact that would give us Auspex 1, which instantly rises one of our senses to superhuman level. If it is something that a mundane human can theoretically notice, then we wouldn't need to roll for it.
Also, funnily enough, dice difficulty adjusters are also relevant here; BSM Molly sees further and hears better after pouring a bottle of water on her head.