The existing stunts that are comparable are:
No Pain, No Gain: You can take a bunch of punishment before it starts to add up. You may take one additional mild physical consequence (page 203).
(Analysis: 2 shifts of benefit useable at most every other scene)
Resilient Self-Image: Your sense of self is strong, enabling you to endure more psychological punishment than most. When facing torture or other extreme interrogation techniques, You may take two additional mild mental consequences (page 203).
(Analysis: 4 shifts of benefit useable only very, very rarely)
So first off, I'd point out that gaining two widely applicable consequences is beyond the scope of a stunt. You can either take one consequence that can be used under common circumstances or two consequences that can be used rarely. I would not consider 'useable with black magic' to be 'rare' for someone who practices black magic (or who deals with such a person), for example. Use the 'Self-Image' example as a guideline: if you plan on making use of the consequence more often that ... oh, say even a CIA covert agent might be likely to make use of the Self-Image stunt, then you probably only get one consequence, tops.
But to answer your question, I would say that you should not be able to stack more than one stunt's worth of conseqences of each type, plus any you gain from your relevant skills. (Note that I would count the mild consequence from your "Absolute Conviction" as not stacking with other stunts, since it is still stunt-based.) I'd still allow overlapping stunts -- for example, a character could take "Deep Reserves" (+1 mild mental) and "Resilient Self Image" (+2 mild mentals vs torture) which would net them a general-use mild mental and a second torture-only mild mental.
Of course, the last paragraph is my opinion on implementing the 'non-stacking' guidelines given for stunt-creation. Implementation varies by game, but note that the rules suggest that any additional stacking stunts should be less effective (and I don't think adding in 'half-mild' consequences is a good idea.)