1 - the pump is still below the radiator in that config so air will be caught in the radiator instead of in the pump. If you were to mount it to the bottom, the pump still isn't the highest point in the system since the block would be above the pump. It's a clever design which basically prevents the issues most AIO's have with pump/block combos in that orientation. (it would still be back to have air trapped in the block for cooling but the pump would still be fine)
2 - any fan with decent static pressure (basically all radiator fans) will work well on any other radiator in the AIO space. When you start to get into custom loops and change fin density you can get better performance from a high airflow fan and low fin density, or a high pressure fan and high fin density. Most AIOs are in the high fin density group and should have high pressure fans.