Its important to put the caveat:
The DragonFlask is Great, its just that it's great for people that use Bright magic or no magic or are not able to do Battle magic.
Lady Mathy doesn't full under those groups.
edit: that said: The Candle of Cleansing Radiance has been waiting to see its day for a long time, and now that we have our own Light wizerds and a Jade Girlfreind, it might have missed its chance until we know we are up against Nurgle or clan Pastalince
To be fair, I think that I've talked myself into preferring to replace the candle over the flask.
It's even more situational then the flask.
The Ring isn't worth swapping anything out for in my opinion, for the simple fact that it does not open up any new options and provides a negligible enhancement
Ironically I think I've reached the same conclusion about the Ring that you had about the Flask
It's gimmick is being a hidden laser gun for surprise attacks, which is pretty decent... for someone who isn't a Grey Wizard of Mathilde's caliber
In virtually every situation where Mathilde wants to sneakily shoot someone, the ideal use for the Ring of Sudden Dawn, she already has a tool that does the job just as well or better
Cloak activity can be trivially cast for her, it's only visual, but unless Mathilde is giving the target a hug that's usually going to be good enough and from there:
She can just quick draw her revolvers, which is the simplest option with the least room for error when a mundane bullet will do the job
She can cast Shadow Knives, which has several advantages on Radiant Gaze; it scales with magical ability and mastery so it hits significantly harder, it ventilates the target with multiple knives so it has a better chance of striking a vital organ or multiple vital organs, it bypasses mundane armor and thin cover, and it can be chained with Smoke and Mirrors
If she's in arms reach she can put that Branulhune quick draw training to use
If she absolutely, positively wants whatever she's assassinating to die and doesn't care about gratuitous overkill she can take a swig of the Dragonflask
And this isn't considering options like full on invisibility, or illusory body doubles
The Ring of Sudden Dawn doesn't bring anything new to the table, and doesn't make Mathilde appreciably better at what she already does
If we really feel like covering her bases on being able to assassinate someone while standing next to them in full view then we'd be better off finally finishing that Cloak Activity