I have been considering some alterations for the mercurial magic system... I love it, but it has had some moments that made it feel a little too "heavy" in delivery... specifically when a Wizard managed to have someone he knows die every time he casts read magic, and an Elf can only invoke his patron while submersed in water.
I consider these things to be a feature, not a bug.
In your examples, an imaginative player can make his Wizard a "Chatty Kathy" who gets to know everybody. I mean, literally everybody
he can. He needs the most Friends on Facebook, 'cause he keeps losing them. Or perhaps he befriends nobody but the man he truly wishes to die, who is so powerful that he can be defeated in no other way.
For Invoke Patron, with characters that have more than one Patron, I would roll a new Mercurial each time, so that the result is specific to invoking that specific patron. Can only summon Straasha when submerged? Try obtaining the patronage of Filyeet as well!
Consider also how the really bad results are the result of low Luck at the time the spell is learned. Take that away, and the Wizard is less likely to consider Spellburn and more likely to simply burn Luck. Does this mean that some spells are unlikely to be cast? Yeah, probably. But, given how game-changingly powerful spells can be, this is more flavour than anything else.
Finally, want the chance to change your Mercurial Magic roll? The game already gives you an answer: Quest For It!
The scenario I am currently working on for Purple Duck Games is a quick answer for Judges who have players that want to Quest For It. In Through the Cotillion of Hours
, characters have a chance to petition Somnos, the Dreaming God, for some specific favour....if only they can reach him before the thirteenth chime rings!
I strongly believe that, rather than allowing multiple rolls or choices (for that way optimization lies!) let the PCs do something to change the way things are! Let them find the Wheel of Fate and spin it, or disturb the Well at the World's End so that all Mercurial Magic results are changed within the campaign milieu. Let them seek out the demon lord of toads and make a compact so that he prevents the toad-rain from falling whenever that spell is cast.
consider making new tables every so often, so that unpredictable results can still occur as your game progresses. Just try to make the bad as bad, and the good as good, as the random result for a given value on the original table.
Have fun with it, and encourage your players to do likewise. How they deal with their Mercurial Magic results is up to them; it is not something the Judge needs to fix for them.