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I've tried cleaning it, flooding the whole area with solder and sucking it off, cleaned it with acetone, scraped it with the point of a knife, it just doesn't want to play.
Gently scraping or sanding is a good idea. Also plenty of flux and repeatedly soldering/sucking. Acetone doesn't help.
You likely have an oxidized layer on top of the copper. This can happen if the iron is too hot: the flux vapors away quickly, exposing the copper to air (oxygen).
Lowering temperature and a good chunk of patience helps. Flux can de-oxidize, but does so slowly. Scratches give you small islands of blank copper, but the surrounding oxide still prevents a good junction, so they're a starting point only.
Another trap is, the copper stops to stick onto the board's base material if it gets heated to hot for an extended time. In case it starts to come off it's usually time to give up on this joint.
A brute force attack is to use plumbers flux. That's a flux with acid inside, so it works much better, but you also have to carefully wash it off after soldering.
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Should I replace the cap, not trim the lead and use it to bridge the gap?
A possible solution. Another one would be to solder a piece of wire into place.