An ILL/IA$ is a purchase. And like most purchases, is ordinarily considered final. If you spent two hours in the park and then left because you started to feel ill, should you get a refund for your day-ticket? How about a partial refund after 5 hours in the park?
Of course, a CS CM may take pity on one's plight and provide a little pixie dust of magic and grant a refund tho they aren't obliged to. And one may get a refund for an ILL/IA$ purchase scheduled for 5 PM if you're leaving the park at 10 AM because you're sick. But they definitely don't want people cancelling and shuffling around ILL/IA$s like they can for DG+ LLs.
Yes, they can be stacked.
Yes, you can grab your two ILLs/IA$s starting at 7 AM if you're onsite.
This has been confirmed by @scottgustin on Twitter, who's a journalist that covers Disney and has been right about almost everything he's tweeted.
Genie+/IA$ Info Summary Link Here