This all looks reasonable until we have a healing / drain spell on an undead who is both weak to and absorbs an element. After UR flips the sign, what's next? Do we always apply absorb first and then weak? Vice-Versa? Or do we manipulate the order to always damage the target? Or should the target always be healed instead?
The problem is that Absorb and Weak are essentially complete opposites, so combining them is a bit of an oddball (which I forgot, because derpy me didn't think about the two being able to coexist). Some possible outcomes could be:
1). Cancel both and make the resistance Neutral.
Sort of a "can't stack" situation, similar to haste and slow. Would be a way for a person to cancel their weakness by adding absorption equipment (rather than stacking effects), or a way to cancel an enemy's absorb effect by inflicting elemental weakness.
2). Combine them to enhance the effect of Absorb (higher elemental effect due to weakness, but still absorbing).
Since there's already a way to cause reduced absorption (Absorb + Half), this would be a way to potentially boost the Absorb effect (Absorb * 150% or so). Could be overpowered though, but could also be useful on enemy units.
3). Skip checking Weak if the unit has Absorb, since Absorb takes priority (or vice versa).
Not the best solution really, but would allow either the Weak effect or the Absorb effect, but not both. Kind of a way to prevent potential OPness of 2 while still allowing some effect, but I'd rather see one of the other options used.
4). Have the same effect as Absorb + Half, but able to stack with Half (potential quarter absorption).
Another possibility, this time making weakness essentially causing your Absorb "skill" to be reduced due to having lower affinity with the element... or something. Since weakness normally causes additional damage, having it reduce the beneficial side of things (essentially giving or taking 50% depending on the situation) could be a decent solution.
Overall, I think I'd prefer either 1 or 4 as possible solutions, with 1 probably being the easy way out while still having potential tactical use if there's weakness-inducing abilities like Oil, or to neutralize a weakness from one piece of equipment/the character's job with another piece of equipment. There's obviously lots of stuff that could be done with elements, and all of them have their ups and downs, so which one's the "best" is kind of hard to decide.