Grave Guardians are peaceful mobs and as such, they shouldn't be blocked. Moreover, you can trade with them. That's another important reason not to block them.

Not really a modpack issue.

You can suggest it to the creator of the megatorch here:
https://github.com/Xalcon/TorchMaster/issues

Why would a megatorch creator care about an unrelated mod compatibility. Isn't it up to the modpack team to sync mods, solve conflicts where possible?

Megatorch seems simple enough to me for the ATM team to be able to tinker it as they see fit.

I'll leave a suggestion there, tho I feel it belongs here.

You might be right. It also seems like there is a config to override the megatorch/flarelantern to allow or block mobs, so this should indeed be simple enough.