The fact that Cassia ends up being a load-bearing part of your line-up is actually insane from a rp perspective. Taking your only navigator into danger is really the last thing you should be doing.
Cassia isn't even that bad, they can do warp travel without a Navigator but it's much slower (and only suicidal if in a warp storm or when trying to match a Navigator's speed). Pasqual is probably even more vital than her, and don't get me started on the Rogue Trader.
With no legitimate heirs, no viable replacement (the officers can hold the fort for a while but only because they managed to suppress the news that the Lord Captain is missing) and a sub-sector that contains tens of billions of souls (if not more!) to look after.
We like to hold Abelard Werserian up as a model first officer because he's very cool, not that much of a prick by Imperium standards, and so on; but if a certain William Tiberius Riker saw what he lets his (Lord) Captain get up to, it would be Abelard's and the RT's turns to get reamed out like fresh-faced cadets.
Especially once Riker found out that the Flagship holds trained and armed personnel who are just perfect for potentially dangerous away teams, some of whom don't even have other responsibilities -- in their hundreds if not thousands.
Yeah, this is absolutely true about the Rogue Trader. I do think Pasqual is replaceable, you have plenty of other Mechanicus on the ship even if they're not as good. And you also don't need him in the party, you can spec other companions to do his jobs.
But as far as Cassia is concerned, the warp is canonically a mess in the Koronus Expanse, so you do need her safe as a navigator. On the other hand, she is so good as both damage and crowd control in a fight, that she almost immediately becomes load-bearing. (That stun on her eyecone has saved my skin so many times, and together with the staff that lets her attack twice, it's insane.)
