You may be able to mitigate some of the ratio differences if you also swapped the rear diff gearing at the same time. I haven't done the math, but that's a common thing to do.
17 or 18:
https://www.fordpartsgiant.com/parts-list/2024-ford-mustang/fender-components.html?filter=(r=1816;e=1755)&diagram=JC24150&partNumbers=PR3Z-16103-B
It works right, in that it cools the oil, but the computer's algorithm for calculating the temp can't be updated. There is no actual oil temp sensor, so manipulating the algorithm is needed.
Sure, unless you're in a northern state and plan to drive it all year, then you'll need to buy some...
Right. Lots of steering means you shouldn't have a lot of throttle. In a turn, as you unwind the steering wheel, you can gradually apply more gas. Gas and Steering are inverse inputs.