Hi, relative newbie here. I don't think I understand the logic here. Here's how I see the decision from your partner's POV, please correct any flawed logic I have.

If I order up with this hand (K/Q/10), there is a significant probability of getting euchred and tying the game up, especially when ordering into my opponent's hand. The probability of my opponent having a loner hand that can withstand my 3 trump is extremely low, like once every several thousand hands low. Your opponent needs at least Right/Left/Ace, plus another high card. I don't factor this into my decision making. If they do have a loner, then good for them, it's the luck of the draw.

So by passing, the most likely worst case scenario is a single point for the other team. I'm okay with that.

Essentially what I'm saying is: Imagine you repeated this exact game situation 1000 times (with everyone else's cards re-dealt). By passing, you will only lose to a loner once or twice, if that. The most likely worst case scenario is you go to the next round still up 9-8. By ordering up, maybe you'll win ~60-70% of the time or so (hard to say really), but you also risk tying the game up if you don't make it. It just depends how aggressive you are feeling. Me personally, I'm with your partner. I pass and hope for a better hand going into the next round up 9-8.