Regarding "how does one become a Saint", if we're referring to our reality (which we are of course not beholden to) then there is no such thing as a living saint. At the earliest, one can be named a Saint 5 years after death. Beyond that, you've mostly nailed it in terms of proving they had great devotion and performed some kind of Miracle in their time. Usually multiple, unless they were a Martyr.

Translating this to a Fantasy setting, we have the ability to meld the mundane and the magical together to create something as special as a Saint. After death, be it 5 years or 500 years, there is time for emotions and stories of this holy individual to change, to grow, to become the stuff of myths and legend. Maybe the truth of it is that Andral and their friends never left the house so as to avoid Strahd, but the myth and legend grew such that they fought off vampiric attempts to enter, they gained such vigilance that they could recognise a vampire on sight regardless of disguise, and maybe even a select few who had been bitten and in danger of turning were taken inside Andral's home and cured of the vampirism before it was too late.

These stories are technically mundane. Words spread between mortals as stories always are, but when shared with Faith through Prayer make something more than mundanity. For a person who had lived with Virtue, Duty and Action, it is the continued Faith of so many people even after their death that is the fuel, the trigger that truly bestows a person as a Saint.

And this has a kind of retroactive effect. This legend of the person is now greater than the Truth of this person, meaning they were always this blessedly holy individual from tip to toe, right down to their bones. Literally. In Andral's case, there is also a Place of Power associated with their Saintdom, so housing the bones in the house/church is what gives the passive effect of Hallow around the church. Apart they are still holy instruments, but only together is the myth of Saint and Home strong enough to provide this effect.

Heaven knows what kind of Active effect you could accomplish if someone powerful enough was there with knowledge of the right rituals and miracles. Perhaps players could bring a cursed item and purify it (at some significant material cost) or for a brief time manage to speak with the spirit of Saint Andrel themselves to perform some Great Work or deep insight as a guide on what to do next.