Interesting challenge. Personally, looking at the room dimensions and your desire for a fairly large TV, I'd probably just take the "risk" and go with a TV above the fireplace. I've never personally hear of anyone having major issues with such an arrangement. If you were that worried, you could perhaps build some sort of a "frame" which sits out further than the TV to deflect any heat away.
Alternatively, consider an arrangement like the first of Minnie's recent pics, where the cupboard part of the bookcase arrangement is actually an angled cupboard (my preference over L-shaped cupboards) to give you more width to play with. Your radiator will be the limiting factor, but I think you should have some decent width.
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