Can I Use an Early Upgrade on a Financed iPhone with a Deactivated Line?

I currently have two active Xfinity Mobile lines on a shared 3GB data plan. Both phones on those lines are completely paid off.

Last year, I added a third line for a family member and purchased an iPhone 15 on a 36-month device payment plan. The family member subsequently got their own service, returned the iPhone 15 to me, and I deactivated that line. However, I'm still making the monthly device payments.

The iPhone 15 is in perfect condition and has been factory reset. Its original purchase price was $629.99, and I currently owe $420.11, with 24 payments remaining.

I'm now interested in upgrading my personal iPhone 13 to a newer iPhone, but I would like to keep my existing shared-data plan rather than switch to an unlimited plan.

My questions are:

1. Is the iPhone 15 eligible for Xfinity's Early Upgrade program even though the line associated with it has been deactivated?

2. If I pay the additional amount needed to reach 50% of the original device price, can I return the iPhone 15 and have the remaining balance forgiven?

3. Can I apply that early upgrade toward purchasing a new iPhone for one of my existing active lines, rather than reactivating the canceled line?

4. Would I be able to keep my current shared-data plan?

5. Alternatively, is there any way to trade in both my paid-off iPhone 13 and the financed iPhone 15 toward the purchase of one new phone?

My goal is to upgrade my personal phone while getting out from under the payments on the unused iPhone 15, if possible, without increasing my monthly service costs.

I'd appreciate any clarification on what options are available under Xfinity Mobile's policies. Thank you!