I own a pair of DT990 Pro 250Ohm and they are awesome.
I use them with an USB DAC and they work flawlessly, so I'm guessing the DT770 should work on your mobo without a problem.
Consoles have a locked internal render resolution for each game, even if you output a 720 signal the console will still render it at 1080 internally, so the performance will not change.
If you are going to use it for 5 years it is not worth trying to save 40 euros now since you might regret it down the line, get the full hd laptop with a 860 since that will provide the best experience.
Here you can see a review of the GPU.
http://www.notebookcheck.net/NVIDIA-Quadro-NVS-3100M.24738.0.html
It seems like it can play pre 2009 games on 1024x768 on low settings.
I have a pair of Beyerdynamic DT990 PRO and use them with a FiiO Olympus E10k USB DAC and they work perfectly. It was a nice improvement over my integrated sound card and it's not that expensive.
Any laptop with a discrete GPU like GTX 860m/960m will run LoL with no problems, but laptops usually have 60Hz screens so even if you get around 100fps you won't be able to see more than 60fps.
The r5 230m is better but it's not a powerful card at all, not suited for 1080p gaming.
The intel hd 4400 should provide a better batery life and it's enough for everything except gaming.
Yes, the system swaps memory to your hard disk when you fill the RAM, but since hard disks are quite slow compared with RAM this process slows the system a bit and makes it feel unresponsive, so it's best to have some headroom.
A modern i3 would be more than enough, it would also be a good idea to get 6-8GB of RAM to make the system more responsive with many things open.
The audio card only affects audio quality, but it won't affect how responsive the system is.