cvt tranny fluid/good tire recommendation

I have an 07 versa with auto cvt tranny with 68k miles.

I did my cvt fluid change at nissan at 60k.

Recently, it was slipping when driving uphill in the cold. I swung by nissan and they said it was a quart low and topped it off.

Its fine now. Should I have the fluid changed again or leave it? Was it likely the fluid was dirtied or damaged when it was low and the tranny slipped?

My understanding is nissan covers the cvt for 10 years 100k, and thus I have 3 or 4 more years of warranty on it. I'm going to take it in this week to have it inspected for possible leaks or faulty parts. I don't see why it would of had low tranny fluid. If there was a leak, does the nissan warranty cover that? I also have the nissan top of the line extended warranty also. It was gold or platinum to cover the car to 100k or 7 years.

Also, I need new tires. What is my best bang for my buck? I live in cold country, and drive mostly in the city, and occasional highway. I don't want to spend more than $400 for all 4.

Thanks