Just trying to figure out how to get coverage on a ticket I'd like to get using a voucher from COVID days.

Full facts: Was supposed to fly CVG-SAT on UA in mid-March 2020, right as things were starting to get bad, but cancelations were still light then. Reason for our trip was canceled, so we canceled flights and took a credit, even though our flights weren't canceled. If it matters, flights were purchased on CSR. This credit was using the same PNR, but we got that converted to a United ETC.

We'd like to use the ETC to book flights to HI, but would also like to have the card trip cancelation/interruption/delay benefits (we have young kids...and let's just say we're experienced in not assuming our trips won't be canceled last minute due to some sort of illness). My understanding is that only a portion of the flight costs need to go on the card to cover us with these trip benefits. Flights are less than value of the voucher, and unlike some other United payment methods (like TravelBank), I can't choose to use only a portion of them to the flight..it needs to be all or nothing. In the past, I have paid a portion of the fare from flights I have used TravelBank $ for in 'cash' using CSR, in order to get the travel benefits.

So a couple of questions on this:
- I assume because this credit was converted from the FFC on the original CVG-SAT PNR, me using the card on the initial tickets has no standing, and so the benefits would not apply - is this correct?
- assuming the above is correct, would making an ancillary purchase on these tickets be enough to apply the insurance benefits based on 'a portion' being paid with the card? For example, if I bought one of the packages they offer at checkout (E+/bags/lounge access, etc.), would that be enough to trigger the coverage? What about a post-purchase ancillary - for example, a post-purchase upgrade to F or even just buying E+ seating?
- If not, is there any other way to trigger the coverage while still being able to use the credit?

Appreciate any insight into this.