I would use proper penetrating oil not WD40, if the issue is purely that screws will not unscrew, and left for 24 hours. Then use the correct 4mm hex (allen) socket on screws. Failing that drilling off screw heads then drilling out remaining thread to allow re-tap may be only course of action.
It appears though that one of the tapped welded in chassis plugs has loosened so you may need to get that re-welded in place.
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