The NPC Djinn Dew which uses Mercury-based attacks has the following resistance:
Mercury 193
Venus/Jupiter 100/100
Mars 25
This is the same for elemental enemies like a Hydra, which uses Mercury-based attacks. With no Djinn set, resistance levels vary slightly for playable adepts. For a Mercury adept:
Mercury 114
Venus 88
Mars 87
Jupiter 86
The same is true for Elemental Power (i.e. Mercury adept strongest with Mercury and weakest with Jupiter). It is weird the playable characters don't follow the same pattern as NPCs.
For some NPC adepts, it follows the first pattern. Remember the final boss in GS2 was a dragon like the game before. In this case The Wise One somehow turned three adepts (Venus, Venus, Mars) into it. The resistance was:
Venus 195
Mars 175
Mercury 130
Jupiter 98
This makes sense if you average 2 Venus and 1 Mars NPCs, but not if you average playable adepts. But the game still included some subtle detail: once you cut off the second head (Mars), the dragon loses Mars-abilities and PP restoration - previously it's PP always restores to max as it was near the Mars beacon.
For other NPCs, like Saturos, the resistance stats are:
Mars 175
Venus 127
Jupiter 100
Mercury 72
His Mars power is 110.
So clearly there are differences between playable adepts, NPCs, and NPC adepts.
The only other evidence of symbiosis I can think of is for dual-element classes between symbiotic elements, we have an extra class level. For example, Brute (Mars-Venus) has 6 levels but Apprentice (Mars-Jupiter) has 5 levels.
Additionally, Isaac restores PP each turn in the Venus lighthouse. Garet does not. However, both Isaac and Garet's PPs are refilled before the Fusion Dragon's fight.
I don't know. It could be due to game limitations or the developers didn't want to weaken your character on the final boss? If they weakened Garet in Mercury, they would have to weaken Ivan in Venus. Or...
Maybe coming from the Mars clan and living in Proxian conditions made Saturos and Menardi more susceptible to elemental relationships? Whereas in Vale the adepts are near Mt Aleph and all 4 elemental stars, so are more sturdy against any elemental relationships and it takes longer for their effect to work? That's why Garet restored full PP in Venus lighthouse but the symbiotic relationship only happened waaaay later. Though this doesn't explain why Piers, born in watery Lemuria, or Mia, born in cold Imil, are not affected by the Mars lighthouse.
So yes there are a ton of inconsistencies and I'm going to put this down to a lack of development time or difficulty coding all this.