I wonder if there isn't an entirely different line here? What if you DON'T Force the Thorn in the first place?
On your turn you can dodge the question of Preordain vs Tundra and play Wasteland + Ruby. That lets you pass with a Force up ... it opens your own Wasteland to his, but that's completely fine - you still end up killing one of his lands, but you're tempo-positive on the exchange, and he only has 2 mana to play something scary (which can't be another Sphere through his own Thorn) if he actually does, you still have Force. You could play fetch-island-ruby instead, but I think playing Wasteland here is better for those reasons, and plays around Strip Mine while you're at it.
So assuming he doesn't Wasteland, he gets to play one or two threats, and you have the Force for the scarier one. Worst case scenario - he leads with a Sphere, so you HAVE to counter (because you won't be able to counter the next threat), and then plays something scary for 2 ... but in that situation you're no worse off than if you Forced the first Thorn - you end up in the exact same situation (1 sphere, 1 threat), because you wouldn't have had the answer to his second sphere.
If you had made that play against the actual hand your opponent had, the following would have happened.
shops turn 1 - Shops player plays Tomb and Thorn.
mentor turn 1 - You play Wasteland, Mox Ruby
shops turn 2 - Shops player plays Workshop, Sol Ring, Ravager, and a 2/2 Ballista - you tap the Ruby to Force of Will the Ballista.
mentor turn 2 - play Fetchland, cast a turn 2 Mentor through a Thorn you don't care about, with a Preordain in hand - likely enough to handle a Revoker, Tangle Wire, and a small Ravager.
Alternatively, if he had the Revoker in hand from the beginning, he would, on his turn 2, Lead with Revoker->Ruby, which you would be forced to counter, and follow that up with Sol Ring, Ravager, Ballista @ 1. In that scenario you still get a turn 2 Mentor, which he is probably forced to kill by sacking the Ravager to the Ballista. You'd rather have the Mentor in play, but in this scenario he has 0 ravager, 0 ballista, 0 revoker, and you have 3 mana ... the alternative play of "Force your Thorn>Waste your Tomb still results in no Mentor on your board ... but you have 2 less mana and he has 3 more threats.
This is easy for me to say with the hindsight of what your opponent's hand was, but I think for most possible hands you end up the same or better with that sequence.
I don't like Wasteland here, but for reasons people haven't brought up yet. A turn 1 seat should be setting off alarms in your head that your opponent is on an Outcome deck. Ignore the fact that your opponent may or may not have a Tolarian, the issue here is not keeping up a Flusterstorm when you had the option to.
I think you're overthinking his Misstep. Hitting your Misstep doesn't necessarily mean the Ponder is important, it could just mean that he like using Misstep aggressively ... or he has another Misstep in hand ... or he has a more important one drop (like the Divining Top or an Ancestral) and wants to take any other Missteps out of your hand before you can untap with Misstep+Fluster in the same turn.
It could even be that the the Ponder WAS important to his hand, but because he had too MUCH mana and not too few - in which case Wastelanding him here actively makes his hand better. Outcome has a low land count, but it doesn't have a low mana count, and unlike the Shops player in the previous game, he had no reason to play out any artifact mana before passing, so you didn't really have any objective information that he was short on mana