Keep in mind those charges listed in #5 generally apply to
changes in your trip, not requests for vouchers because of lower fares, but keeping same ticketing. I do know I have received vouchers, no cost, in past few months from both United and Midwest (Midwest last week).
I do know also that American is one that does charge, $100, for the voucher, as they consider it a fare change.

I had booked them for an Orlando trip some months back, then they lowered the fare. I called for a voucher, they said, $100 (for about a $30 reduction), so obviously I said no. Then, a few weeks after that, they changed the times significantly, I called and cancelled, full refund, rebooked Midwest, and then a subsequent voucher there after they lowered. It pays to periodically check fares, even after booking, with most airlines.