I have similar solution so I can tell
a) Flickering is not visible at all. Not even close to low frequency monitors we had before LCD come (maybe you remember that times)
b ) lifetime of led should not be afected, at least could be ignored (if you will replace LED after 5 or 7 years ....it doesn't matter much, since it will last much more than traditional bulbs)
c) Power consumption is current times voltage. Voltage is constant, and current if at 100% is for maximum brigtness. If you use arduino PWM then 50% means that at 50% you have current, and vice versa. Effectively it means half the current. And that means half the power. So your assumption is correct.
MOSFET has incredibly low resistance, so almost nothing is spent on MOSFET (comparing with traditional transistors as your question might imply)
d) Input power must cover your LEDs. If you use 50W LED than you should have at least 50W/12V = 4,16 A for maximum load. With some reserve i would probably take at least 5A supply. Real answer is more - how long you will really have 100% power.
In my project I usually pick some color so even if red is 100, and blue is 100, maybe green is 0%, or .... So in my case average is ~50% of maximum calculated power most of the time.
TO be on safe side pick 8A 12V supply (conservative) or take 5A 12V supply which in my case is more than enough because I almost never use white collor (all collors at maximum).
- MOSFET cooling. It is incredibly low heat on MOSFET. In fact my power converter 12V to 5V for electronic is hot, while MOSFETS are incredibly cold. Again the fact that internal restistance is very low, helps a lot. I created a board with place for coolers. and I bought passive coolers (similar to these on link you provided) . But after testing I decided that that makes no sense. So I didn't put them on!
I am talking about my project expirience, where I use 5m 5050 RGB LED, with 8266 microcontroller and 3 MOSFETs - 1 for each color. I use 5A 12V power suply, and swithching module for adapting power for my 8266 microcontroler (otherwise power converter overheats).
I hope my expirience helps.