Some things:
-for the case of a rollover, I typically pay some portion by credit card (e.g., on a USD800 ticket to europe I'd pay 600 by giftcard). They should then refund the CC portion first (and do if you tell them to).
-I don't think one has to buy small denominations of gcs, because you can specify the amount of gc you want to use - and you can use the remainder for the next booking
-receipt: the electronic receipt online is not worth much, it usually only gives me the itinerary online. But when I check in, I ask for a receipt and they print it on a boarding pass like thingy. There it lists all the fare and tax components and there is one line that has the amount paid by CC on the last line. The GC payment is not detailed. I just circle the amounts making up the overall cost and hand that to the travel dept.
-I don't think paying only a part with CC helps with insurance: in the T&C it says something like the whole fare has to be paid using the CC... But maybe someone has tried and succeeded.