Japan airlines does offer early check-in up to 12 hours early. Your choice if you want to spend hotel night at Conrad, or at airport(landside Crown Plaza/yotelair) , or inside airport (various airside hotels or even no hotel)
https://www.changiairport.com/en/fly...-services.html.
Unfortunately jewel itself doesn't have that much open at 7am, but definitely there are places open in airport for food, what are you in the mood for? Kaya toast, dimsum-lite, nasi lemak?
Jewel, see cafe/restaurant hours, minority at 7am
https://www.jewelchangiairport.com/en/dine.html
Staff canteen would definitely be open for breakfast if you are curious. Cheaper too, since you were thinking of taking MRT to airport: