ベッドから急いで起き上がりアルスの後ろに駆け寄ると開きかけたロキの口をアルスの手が覆った。 首を傾げて何事かと思ったら、勢いよく扉をスライドさせ――。 Loki was about to speak a crime she committed, so Arus jumps up to stop her. The translation I was able to come up with goes: "Loki hurries out of her bed and rushes after Arus's back. Just as she opens her mouth, Arus covers it with his hand." I'm not sure I understood this properly. Did I?? Then the following line completely loses me. "While tilting her head to think, someone forcefully slides the door open." Did I understand this part correctly? Thank you for your help~
If I were to make a wild guess (almost zero Japanese, aside from the stuff I know watching too much of it ofc)... "Just as she was about to say it, Arus jumped to his feet and covered her mouth from behind her." "While tilting her head and thinking what the hell was he doing, the room door suddenly/forcefully slided open."
From looking at the raw, Arus is about to head out the door and is beckoning for Loki to follow. Your understanding isn't terrible but your translation is way too literal. ベッドから急いで起き上がりアルスの後ろに駆け寄ると開きかけたロキの口をアルスの手が覆った。 → When Loki leaped from the bed and ran up behind Arus, he covered her half-opened mouth with his hand. (The second half is literally "Arus' hand covered Loki's half-opened mouth". Hands doing things on their own sounds weird in English, so it’s better to switch up the subject to Arus himself. The second one you might be able to understand better if you translated it more literally first: "When (she) tilted her head and wondered 'what's up?', (someone) slid the door open forcefully, and...." So she's not merely thinking, she's going "WTF?!" right as the door bangs open. 首を傾げて何事かと思ったら、勢いよく扉をスライドさせ――。 → Just as she cocked her head in puzzlement, the door slammed open. (I'd drop the trailing part of this sentence because it stands out way more in English.)