"I know this is unpleasant, but it's important.
Your beliefs about Hell, from what I skimmed in the brief, are mostly accurate, but irrelevant for you. You got very lucky by accidentally meeting the requirements to belong to my god, Marra. She's new and obscure and much cooler than Asmodeus, who rules most of Hell, the parts you've heard of. She will let you keep almost everything you like about yourself, and as much of your memory as the average angel or agathion keeps.
Asmodeus only lets people be happy in narrow ways, which in practice means that devils are usually miserable. Marra is indifferent to most happiness and actively in favor of vanity. Also, once you definitely belong to Her, She will make sure you succeed at becoming something awesome.
The Abyss is worse than you think. You can't just 'wander forever in the dark'; it's eat or be eaten, and both are unpleasant. You might find a demon lord to follow who will protect you and make you awesome, but probably not. You might obtain followers of your own who you can shape to be awesome, like your mice, but almost certainly not. Marra can give you a guarantee on both.
If you agree to push for Law, if you're found to be Evil, then if Evil is ruled out I'll argue for your preferred systemic alignment." (Normally she has a duty to argue for Law then, but she's choosing to prioritize the duty of winning outright, which is allowed. In the unlikely case that she's making a mistake, she might be punished for it later, but fairly, in a way which teaches her how to think better and serve Marra better.) "I can read your mind if I touch you, to verify your agreement and learn your preferences - it's slightly better to keep them secret."