The default appeal is hard-coded to 'Appeal-default' in RapidHTML, and to JimmyQuote in GatewayFormChooser. These should both use $wgDonationInterfaceDefaultAppeal. Also, when a different appeal is given on the query string for A/B testing, the gateway should store the value and re-use it on subsequent views in the same session.
Original ticket: Currently the default appeal for payments if nothing is specified in the URL is Appeal-default. This is woefully out of date and contains a number of inaccuracies. We have been linking to JimmyQuote as the appeal from donatewiki and all banners for a long time now. We should make this the new default even if it's not specified in the URL.