If you can wait till Q4 this year, then probably they'll come out with WLED (White LED) backlight eIPS, which some people are not fond to have. Hopefully the color isn't as bad as some people described... *pray hard* xD;;

WLED has been an issue for very long time, which is why even the top-end H-IPS/S-IPS still using CCFL as their primary backlight (Apple exception).
If the WLED is the LG's (can't remember the model, lazy to flip back), kindly need some people to compare the LG one to the current Dell, cause Dell's current IPS is manufactured by LG, is the LG's IPS manufactured by itself as well?

Raise your chair, put a small little stool to put your feet onto comfortable position, that way you won't sit until your whole lower leg goes numb.
And that position is for watching movie? Or working? I think it's fine with watching movie in relax position but not ideal for working.
Distance from the monitor is fine as that, I came a little close when I type this and a little further when I watching movie.
You can consider a move-able table (wheels) that slide between your leg and the table height, there you put your keyboard and mouse, and you can sit further away while still able to reach your keyboard and mouse comfortably.
Usual computer desk has slider to put the keyboard and/or mouse for this purpose.

Are yours DP > HDMI active or passive converter? Normal graphics card does not output DP signal through their HDMI and vice versa. DP and HDMI are 2 distinct technology and different protocol so you need active converter to do so.
Passive converter usually just a 2 heads and plugs, while active converter require some sort of adapter and require power source to power the adapter (5v USB cables, etc.).

I don't think this is an active converter, like I said, normal HDMI output won't carry DP signal so no it won't work out of the box, you need a HDMI > DP active converter to do so, and unfortunately your cable might went to waste, or sell it off to someone whom might find it useful (rare case).