Could a variable sweep aircraft also use negative angle? Variable sweep on fighter aircraft is a bit out of fashion, but seems to a worthwhile option to adjust the wing to the current speed and needed lift. All this is bought with complexity and weight of the system.

Would it be possible to build a variable sweep aircraft that goes low speed and in a dogfight into a forward swept wing configuration like the Su-47 and high speed in a rearward swept wing?

Image of Su-47 credit to Wikipedia (Leonid Faerberg)
Yes it could, but don't hold your breath.

Forward sweep means that the aeroelastic forces will twist the wing at high lift into an increased angle of attack, so such a wing needs to be stiff in order to avoid divergence (PDF!). Also, aileron effectiveness is excellent, so a forward-swept configuration is a good choice for dogfighting.

Since backward sweep is mostly helpful at trans- and supersonic speed, swing wings were the right answer when very high top speeds (Mach 2+) needed to be combined with high loads at take-off and short field length requirements. When flying ever faster went out of fashion in the 1970s, the attractiveness of variable sweep disappeared.

Forward sweep helps both in air combat and in transsonic flight, so there is little advantage in a variable-sweep configuration with forward sweep. An aircraft optimised for air combat has little need to fly Mach 2+, so the moderate sweep angle is a good choice for most of its flight time. Also, the aeroelastic instability increases with sweep angle, so more than moderate forward sweep needs a very stiff and heavy wing.