1 - Some cases have space for a 360mm AIO on front but the bottom fan blows into the PSU shroud area so not overly useful. Its why I have a 240mm AIO on front as at least both fans blow at the area of case above the shroud. I just left bottom slot empty on front.
front of case is ideal place for AIO as it gets cool air from outside and not fed warm air from GPU.
1. Capacity. A 240mm AIO is roughly 250w of cooling potential, pretty much exactly the same as the biggest air coolers. A 360mm AIO is @ 300w of capacity, but to a 65w cpu that'll make negligible or no difference to a 240mm AIO.
2. There are. There's several companies like Corsair and NZXT who use 140mm, 280mm and 420mm rads/fans. The only reason they are not as popular as the 120mm varients is because case designs use 120mm as the basic fan dimension. Can't mount a 140mm fan/rad in a 120mm exhaust location etc.
3. You assume a rad capable location must only be used by rads. Locations are options to mount a rad or a fan or both. Yes, there's plenty of ppl who do in fact use 2x AIO's, there's the one on the cpu and one on the gpu. There's also the possibility of custom liquid loops, so available space for radiator mounting to get the necessary capacity for the loop is of utmost importance. On the flip side, those locations can also be used by a plain fan for airflow in an air-cooled pc that doesn't use an AIO.
Realize that all these coolers are just air coolers, even custom water loops eventually use air blowing across a water filled heat sink to dump air into the room. So all we're really doing is changing the location and size of the radiative surface. CPU coolers are limited by the location of the CPU and the amount of weight the motherboard can realistically handle. AIO's use water as a transport to move the heat from the CPU to a much larger heatsink that is supposed by the case. Custom loop lets you expand the radiative surface to as big as your case allows while also cooling other things like GPUs.
Technically a radiator can be any size, I've seen motorcycle radiators used to cool PC's before, back before everyone decided the 120mm fan was going to be the standard unit with 140mm as the alternative. Radiators need fans and both need to be bolted to the case, so it just makes sense to make them the same size for universal compatibility. 240mm seems to be the most common size, and it makes sense because CPU coolers already come in 120mm sizes, a 120 or 140mm AIO is kind of a waste. 360 is overkill for most every CPU out there, so really not that useful. And no, you shouldn't mix custom loop and AIO. Building a custom loop is something you don't do in half measures, its amazing but fundamentally changes how you approach build and maintenance.