Well, skill points help (a lot at times), but the biggest thing is tactics. Merc mode is fairly easy as well. I really didn't find challenges (aside from a few bosses and a few large mob ambushes) until halfway through Vet mode. So the game seeming easy is no real surprise.
And no, you don't ~have~ to distribute skills to beat the game. They're just an edge.
You have two of the heaviest hitters in the game for dealing damage (Crossbow and Thrown Weapon. Dual-Wield would be a third worthy of mention.) I imagine a lot of your battles ended before your tank got there

With Taar acting as a healer, and dealing out additional damage with combat mage spells, you would have a very effective damage team.
Also, the old saying "Clothes make the man." is big here. Equipment can more than make up for skills. I see your tank has a good array of skills even without distributing any

If you ever played Diablo2, you can easily understand the benefits of items that give class bonuses across the board.
You say you finished the Campaign on Merc. Did you also complete the Broken World on Merc?
As for Taar. I prefer to run with Taar-NM and Finala-CM, and build both up pure. Taar gets some sick damage with ice spells and maxing out healing and buff skills gives the entire party an edge. Though a mixed mage is very versatile, but takes longer to work up (obviously). Since you aren't relying on the mage for damage (Your ranged characters are pulling that weight), I think you would be free to develop the mage as best suits your desire for fun.
As a last note, I'd like to congratulate you on going so far without using skills. While not unique, it does show a very good aptitude for tactics and gameplay.
~Jessy