I have an EVGA SuperNOVA 650 P2 and I have installed it in a Rosewill NAUTILUS ATX MId Tower case.  The case places the PSU at bottom and I installed, as instructed, with the fan facing down. I am sure the idea is that the air under the case is cooler so forcing it through the power supply and out the back will give it greater cooling.  What troubles me is I have never seen a computer case with much clearance at the bottom.  I have always heard that four inches was the run of thumb and no case gives more than an inch or two of clearance on the bottom.  I wonder how functional this will be over the long hall. I have the case set on a table with a hard wooden surface and the room never gets hot still I worry.

Shouldn't be a problem.

Should be fine considering how little heat Platinum PSUs generate, but personally I'd install it fan up for better convection cooling while running in silent ECO-mode.

Your fine. The 4" recommendation is to cover every scenario, such as people who put there rigs on long nap fiber rugs on the floor or throw rugs of weird build etc..... I would def suggest putting some sort of filter on it though, if it doesn't come with one, your PSU is the most important thing in the whole case and the last thing it needs is a bunch of dust stuck in it. I like this type, https://modmymods.com/phobya-fan-filter-120mm-black.html   because it screws a backing on to the fan and the cover and filter snap on that backing so when you have to clean it it is tooless and easy. They have magnetic filters that stick to your case, if its metal by the fans, like this http://www.newegg.com/Product/Product.aspx?Item=9SIA85V40T5813&cm_re=120mm_fan_filters-_-9SIA85V40T5813-_-Product  , but I wont use them myself because I don't want to put any magnetic fields near my components. Yes, I know its probably not going to hurt anything and its a very weak field but my paranoia wont let me take any chances on my main rig.