I’ve read on multiple forums, that a “+ to all skills” equipment will raise a merc’s aura level as well. (from a runeword, like insight)
I tried it with a tarnhelm (+1 to all skills), and “regenerate mana” doesnt changed at all.
I went outside town, even tried it with my main char equipping the polearm and helm, mana regen doesn’t change at all.
There was a bug in the advanced stats page resource regeneration display where +skills for warmth wasn’t displaying properly, but was correctly taken into account based on my own testing.
It seems likely that this would fall into that same bug and you likely are receiving the correct bonus.
You could test it by spamming a skill until you run out of mana and timing how long it takes the mana bulb to refill with and without the skills bonus to see the difference.
Auras from items never benefitted from +skills even on your character let alone your merc… only way to increase it is to wear multiple items with the same aura (ie Dream helm + Dream shield). +skills on Merc will only affect his personal aura (Might, Holy Freeze, Prayer, Defiance, etc,)