If we do want to hire someone from the college to help manage EIC, I rather have them being a perpetual apprentice than a journeyman. Easier to get and less chance of blowing up/getting possessed/summoning a slaneeshi demon for research.
Depends on what we want them to be capable of.
If we want someone who is capable, if necessary, of going onto an infiltration mission to steal ledgers (c), we want a journeyman.
If we just want a clerk who is incapable of independent field action on our behalf, apprentice is fine too.
I think former is preferable - the map between EIC and K8P is not safe, any journeyman is likely to have to go out and stab a gribbly or deal with corrupted trade guard or check the ledgers(tm) at least at some point. Costs more, but we are paying for the valuable resource, not nothing. But I can see the arguments towards a mere apprentice.
edit:
I've got a real crazy solution for you buddy. I get it's unorthodox but, the daemon could always... wait for it... cut off our hand containing the seed. Never mind the fact the Seed doesn't let us regrow loss of limbs or massive bodily trauma.
As for your 'inability to judge the risk of possession' I'm going to remind you this hypothesized resistance is an incidental benefit- not necessarily anyone's specific priority. As for the risk of possession on miscast... just use common sense, a slight risk in general because who knows with Tzeentch and a significantly higher chance when we're putting our magic and will directly against Daemons.
Is that likely in the near future? Not really, but we're not going to see Avatar any time soon, and it's by no means wasteful to consider defenses against Daemons in Warhammer.
As for Ranald Sorcery? I could give any number of reasons, but in the end- because it's awesome. It gives Mathilde her own unique brand image that stands with the likes of other big name casters of the setting. It gives us a way to have lasting impact in our specialty long after we're gone. It provides awesome narrative potential in exploring where divine magic begins and color magic ends and convincing others to allow it. It's a potential magnum opus borne out of the development and refinement of unique character traits we've had since the start of the quest. It doesn't have to be the theoretical most optimal choice to be gripping and cool, to change how we the voters, Mathilde herself, and the society she comes from view magic can't be anything but compelling.
Lots of things can give her unique brand image.
Assassin + Infiltrator + killy sword, for example, gives a very straightforward brand of "she can sneak up almost anywhere and kill almost anything there ever is", especially if we then double down and develop those traits.
Scout + Hands On gives another brand - a woman you go to if you need to get into some extremely well-guarded/dangerous reliquary, get some information on what's inside and get the priceless data nobody else could possibly have acquired to the people who need to know it.
Collegiate + Warrior of Paperwork is yet another brand - pretty simple, think of that scary Head Bursar woman. Not my fave, but Grey Order certainly has respect for the kind of person who can make nobility shit their pants and pay their taxes purely via managerial/paperwork magic.
And so on. If you put some thought into it, lots of traits and their combinations can give her compelling brand images. Sorcery does not have monopoly on that.
(You could argue it's more impactful than any of those; I would counter, quite reasonably IMO, that with time investment enough to develop Sorcery, if it is possible at all, those splats/brands could be grown into comparably fearsome things)