After much thought, I've come up with opinions on whether we should replace our Candle of Cleansing Radiance and what we should replace it with.
Firstly, yes, I think we should replace it. We got it because at the time, chemical attack by Skryre was a real concern, but right now, chemical weapons and poison aren't as pressing concerns as other things. Someone else can get better use of it, I'm sure.
The single best thing we can replace it with is an item loaded up with Ferment so that we can make Stirlandian beer wherever we go. Sadly, because only Stirland has the proper facilities to fully enjoy it, this idea is impractical.
The second best thing we can replace it with is Starshine...but only for next turn, after which we permanently discard it for something else. It turns off invisibility and finds secret doors and the like, which makes it a pretty good thing for Drakenhof. After that, not worth it. If secrecy weren't a concern, it would've been better to simply hire a Celestial Magister. (Hubert doesn't know the spell.) As is though, Starshine's only 5 CF for the item and that's not bad for a single turn, given the potential rewards.
Illuminate the Edifice would also help, though naturally it'd take up Starshine's slot. It makes the whole building shine with daylight, making it easier to see, which could be of value in the Black Library. (The daylight would've been fantastic if it were still populated by vampires.) It's cheaper at 3 CF, so it'd be even easier to permanently discard once we're done with it. (Possibly 5 CF for the unlimited building size version.) However, unlike with Starshine, we know of a trustworthy light wizard to accompany us: Egrimm. He could cast the spell for us whether or not we pick up Starshine, and he can also watch our back down in the vampire capital of Sylvania.
For a permanent magic item to replace the Candle with, we gotta look at four broad categories of magical items: buff, debuff, kill, and utility. Mathilde's already strong in attack, defence, and healing, so no need for buff. For debuff, she's got Mistfying Miasma. Mathilde's got melee killiness with Branulhune and big killiness with the Dragonflask. That leaves utility.
I've identified two spells that'd be useful to have in our pocket: Wings of Heaven and Burning Vengeance. First gives flight, second causes the target to want to kill a specific other person for a year if they fail a will save. Now, flight is useful, but the mobility is mostly obsoleted by teleportation and Skywalk. Burning Vengeance has very situational use but we can cause some real havoc with it, pitting army leaders against each other or something. So Burning Vengeance is better for us, right? No. Wings of Heaven is better, by a lot.
While Burning Vengeance can be very powerful in the right circumstances, those circumstances are very rare. Not only that, but the will save is a big problem. Most of the people we want to use it on like vampires, warbosses, and wizards, they have strong wills. Skaven commanders might be vulnerable but they already all want to kill each other. For everyone else, risk-benefit mostly dictates an easy Branulhune assassination. Where Burning Vengeance truly shines is civil disruption: getting weak willed but politically powerful human nobles to feud with each other, but that's far from our wheelhouse.
Wings of Heaven on the other hand? Amazing. Perfect. If it were an animal, it would be a very pettable domestic rat. To start with, its specs: several minutes of non-mutative flight at faster-than-elf speeds for 3 CF. Is it obsoleted? Only a bit; Skywalk and Smoke and Mirrors have strong limitations, while Wings of Heaven has its own unique boons.
-Skywalk needs to be cast, and if we're casting it, we're not casting something else. It only lasts a few seconds, so we'd need to be casting it a lot to stay in the air for anything more than a brief length of time. We're also slower in the air, moving at our usual speed, and it gives us less movement possibilities than true flight.
-Smoke and Mirrors has the same casting exclusivity to an only slightly lesser extent. If we're casting it, we're not casting a Lesser Magic/Relatively Simple/Moderately Complicated spell - Skywalk included, which makes vertical movement clunky. The spell provides instant movement, but it's short ranged, so we'd need to chain cast Fiendishly Complex spells to move long distances with it. Finally, SnM can put us in the air but unlike SW and WoH, it can't keep us there.
-Wings of Heaven is great. We can combo it with SnM for easy mid-air teleportation (easier than SnM+SW), which is great in combat. We can glide across the ground to maintain its superior speed at ground level. It's neither magically nor physically tiring to use. Unlike SW and SnM, it can break our fall no matter our falling speed: a good safety precaution and enables certain feats like jumping out of our gyrocarriage whenever we want no matter its height. Finally, something that's valuable to me: it's excellent quality of life; true flight is very fun and Mathilde would get several minutes of it at a time.
So that's what I think: permanently replace the Candle of Cleansing Radiance with an item of Wings of Heaven, at a cost of 3 CF.