Hi everyone, I wanted to follow up with my outcome after following some of the advice shared here, in case it's helpful for others who may have similar questions in the future.
TL;DR version:
Every one of my questions resolved nicely to my satisfaction and I did NOT have to cancel and rebook after all.
Long version:
First, I did some research on the travel insurance benefits for my Chase Sapphire Reserve credit card. I found the benefits guide online and it turns out that every single one of the travel benefits states that the benefits apply as long as I charge "all or a portion" or "any portion" of the trip to my card. This seemed promising, but I also called Chase to verify. They confirmed that even though I didn't put the deposit on the card, we'll still be eligible for the travel insurance benefits if I put some or all of the remaining balance on the card. First problem solved! I've also downloaded and saved the benefits guide to my trip folder for documentation.
Next, I reached out to HAL via their website chat to inquire about adding AARP onboard credit.
The answer, as I expected, was no, since I verified AFTER making the booking.
I also asked whether I could downgrade my booking to category VH (yes, saving $220/person), and then add Club Orange and select within-category cabin upgrade (yes, at CO cost of $175/person). Kind of a no-brainer to pay less and end up with a better cabin AND CO benefits. However, before making a final decision, I wanted to discuss with my husband since you have to pay for CO upfront.
About an hour later I called HAL (rather than using online chat again) to initiate the downgrade and CO purchase. I am so glad I waited and did this over the phone, because two things happened to my benefit.
1. Downgrading to category VH ended up saving us $260/person (vs. the $220 quoted by the online chat person). So after downgrading (-$260/person) and then adding CO (+$175/person), we ended up $170 ahead in total.
2. I decided to ask again about the AARP onboard credit even though I knew the answer was no. To my surprise, he looked into it, and apparently in the process of mucking around in the booking to downgrade our cabin and then add CO/corresponding cabin upgrade, it had already triggered applying the AARP credit. Sure enough, on the updated confirmation I received via email, we now have a $100 OBC that wasn't there before. I'm still puzzled about why this happened since from everything I've read that should not be the case, but I will not look a gift horse in the mouth.
Moral of the story: Talking to a live person on the phone was worth the extra effort.