It's not NTR in that case if you simply have Belle be the beasts prisoner, and not his lover. Then it's just Hiccup rescuing her. In this instance I think just having Hiccup demonstrate how sexually superior he is in length, girth, staying power, stamina, and load size would be plenty - no need to have a monster who acts like a monster touched Belle.
Say, the beast agrees to whoever is sexually superior should have her as the alpha. Hiccup objects saying that Belle should be free to choose her own lover, but Astrid silences him and says that he accepts. The Beast pulls out his 9 inch gnarled cock, believing hell beat Hiccup. But Hiccups harem caress and stroke his knob to its full 18 inches, making sure Beast cannot miss that Hiccup is twice the man. The Beast masturbating his own prick, while Hiccups harem pleasures his prick for him. When Hiccup proves himself superior in every fashion, Beast loses it and attacks him. Hiccup fights him and defeats him before saying Belle is free to go. Belle says that no, a deal is a deal, and Hiccup proved himself the superior male, so he can claim her.
Also, how do you feel about throwing in Fifield the maid, or the bimbettes (yes, that's really what they're called)? Just because they hold no strategic value is no reason to turn down some already busty and flirty French girls.