OK to replace just one tire?

Hit pothole at 75 mph on I93 northbound. Driver front tire shows bulge in sidewall, indicating cord damage. Tires are excellent, expensive ($221 installed each) Toyo Garit KX snows in stock size. Original tread depth is 12/32; average current tread depth is about 9/32. I've read that all the tire circumferences should be within 1/4". If I replace just the one tire, I'll be over that. But normal tire wear and tire pressure differentials puts many, many Subarus over that tolerance. With an open front differential, the issue would seem to be some (small) stress/heat on the viscous coupling in the center differential. If that's true, replacing a pair wouldn't help and I'd be faced with replacing all four to the tune of $900. That seems crazy. Direct Tire here says they do not know anyone locally (Boston) that shaves tires.

Am I really in trouble by just putting the new tire on and not worrying about it? And if Subaru is really worried about tire circumference, what's with that compact spare?

I wouldn't do it and have advised many not to.

The spare is just that, a spare to get you back on the road to get the car fixed, not to substitute for a lack of replacement.