Well, the obvious counter to archer issues is an Air bless. If you go to Air-9, then Orb Lightning (which might be an issue) is less of a threat.
As for the heretics I recommend, if you want to start a blood economy, you're going to have an awful lot of non-Blood Revelers running around, who just so happen to be Heretic 3. Those aren't half bad, but I usually use them for 'dominion clean up'. I should think that the Fire heretics would be good for combat support as well as Dominion kill. This would also suggest using the Fire resistant national troops or summons as your raiding 'army'.
Can't really comment on the 'Assassination in MP' issue, although considering the minimal investment needed, the cost of the guards for the commanders might be more than the cost of the Satyrs. Economic warfare is still warfare.
Worst case, give them a Bane Venom Charm and send them in. Can commanders pick those up?
Now, the magic questions. The Serpent Cult has _extremely_ versatile magic, with most of it accessible at the 2-level. Air, Earth, and Blood might be a small problem. Don't expect the greatest multipath abilities, though.
Spells? Well, if you mean by 'global', I'm not exactly sure. Your most ubiquitous magic is Water and Nature, so that might give you some leads.
Battlefield magic depends on your gameplan. If you go Hydras, Poison is your friend. Your bestest buddy. Any/all people on the battlefield that you control should be poison immune. If, however, you go for the heretic horde, as stated Fire evocations sound like 'the plan'.
Well, I tend to like sacred mages myself. Your capital should be producing a Serpent Priest a turn as soon as possible. These guys are your one-stop shop, especially for a hydra strategy. Poison immune innately, Holy-2 Priest, and IIRC 80 Leadership (may only be 40).
As far as other mages are concerned, I like using the Renatus for research purposes in the 'interior' of my territory. Basically, non-heretics. I place the heretics on the borders where they can be quickly placed to get rid of dominion. It sort of makes a dominion 'dead zone' that anyone will find extremely hard to penetrate. It also gives them a quick route into enemy territory.
I hope this helps.