To removed the clip, I like to "flatten" the seal a little bit where the ends are just enough to grab the clip with flat nosed pliers.
I've tried all these methods with varying but usually disappointing success. What I generally do now is dig around and pull out the spring and the rubber seal, then go after the metal with a pair of metal shears, visegrips and a long pry bar. Force over finesse, but there's a feeling of satisfaction when it's done. Destroys the seal in the process but so what.I drill a small hole then use a slide hammer with a sheet metal screw.