This is on my phone, so I really can't promise a thorough response.
As I've stated in previous replies the magic does two things to allow this:
1. By stealing a portion of soul essence the scoundrel is capable of mimicking what allows for another holy practitioner to practice it, in this case it is the living flame.
2. The trinket is used as the means of casting the magic, not the scoundrel themselves. Given, due to this the trinket becomes a massive weak spot.
Over the demi ascended I understand what you're saying, and truly personally preferred how the ascended used to simply have different abilities in the silver sect rather than being limited to one thing. A scoundrel is also only capable of casting soulfire, just as a demi-ascended would - yet the scoundrel faces hordes of weaknesses in comparison to the hordes of benefits any ascended receives. While an ascended is immortal a scoundrel must put their life on the like to progress, while an ascended possesses a small regenerative capability a scoundrel becomes weak to ascended magic while casting and if their trinket is broken they are sent unconscious, while an ascended receives a strong barrier about their soul a scoundrel progressively loses willpower and becomes a creature of greed.