So firstly it sounds like a really interesting idea and I know people have had fun with the Dawnish version of this game (replace virtue with Love.)
A few things to consider:
You will have a fairly limited pool of resource to curse at an event based on how many mana sites you have in the group, at most you can do 2.3 curses per player with a mana site. That will reduce with more powerful curses, that said I assume with the investigating and meddling that will come with the concept it would lead to plenty of game it’s worth bearing in mind when building characters.
Political Power in Highguard comes from congregations and some of the powers of a priest might be useful in your mission. Particularly raising judgements of Inquisition.
Realm, this is an important thing to decide for a coven since it determines what rituals you can cast, at most as new players you could reach group number*8 Magnitude without support and that is if you have spent all your exp on one realm. (There are ways of getting higher but having things that are firmly in your range is good planning.) The other issue is each of you will have 7 mana if you have mana sites (a ritual if mastered costs 1 mana per 2 magnitude.)
As such I would definitely recommend having a realm with lower mag curses. The higher magnitude curses you go for the less you can afford. Being able to cast a curse alone will also mean you will struggle less with drop outs and can do it on a whim.
Winter: Personally I think Winter is the best option by far
Winter has the lowest castable curse (at mag 6), it also has a few options at Mag 8 although both are more challenging to cast due to requirements.
Winter also has a range of bigger curses at 20 (loose the effect to benefit from herbs, and all food tastes like ash) 23 (Curse them with Nightmares and can’t regain hero points naturally.) and 40 (weakness for a year.)
Night: Has the other low curse, but unfettered anarchy while really fun, might not fit as well with the concept due to connection to a false virtue. (and might make some unvirtuous folks act less virtuously)
Spring: Spring’s curses are at 15 (Infested with biting insects) and 40 (venom for a year),
Autumn and Day also have curses but Day has no low level ones and Autumn’s Mag 8 requires consent, and it’s more of keep deal that punish curse.