Quick question, and please forgive my ignorance, I know that some (all?) AWD drivetrains don't like having 1 new tire and 3 older ones. If/When I get a flat will getting just a single brand new tire cause issues with the AWD system due to lack of wear?

 

I know some AWD systems are pickier than others and can't comment how Nissan's system is but a simple and common solution is to shave down the new tire so that the height (diameter) or circumference matches the other existing tires. Here's an article from Tire Rack about this.

They actually list the tolerance that Nissan specs for the GT-R as an example.

I don't know how many places have the equipment to shave a tire down either. Can most tire shops do it or is it a specialized thing ? Tire Rack can do it as they refer to in the article.

 

When I get a flat, it's usually a nail. Easy fix with a repair kit.

If it's bad, maybe look for a used 225 55 17 on craigslist for about $30-40 to match your 3 older tires

 

As long as you're using the correct size tire you will have zero issues with the AWD system by putting a single brand new tire on if the other 3 are worn. The only time there's a problem is when the tire size difference is greater than +/- 3% which isn't possible due to only tire wear. You could always measure the tire height to verify.