I wonder, like Gerleathingy already suggested, if the side of the handle has bow'd a bit and just needs a little bit of a tap to push it flat again... not too much though, remember Father Ted...
I agree with Fuzzbucket and Gerleathingy, and think this is the main issue.
There is a bit of a gap (in red) where the handle has bowed out. You can see that this allows the prong (that locks the blade) shut to rise up a little. But I don't think the issue is with the prong (I assume the serrated blade locks into closed position when the handles are open).
If this were
my Wave I would take a flat end screw driver and hammer, and give the handle a good tap
(as indicated on green). I did this to my Wave just now, as one handle is does not 'click' into open position as good as the other handle does (but it's not too bad). Actually, it made no difference.
But maybe I need to hit it harder
Waves are not impossible to take apart, so if the pliers head were removed from the handle I think it would be easier to bend/bash the handle into position.
I DO NOT RECOMMEND YOU DO ANY OF THE ABOVE TO
YOUR WAVE.
As for the gritty inner tools, I had a second hand (almost unused) Charge with gritty tools. A bit of oil and a fair bit of opening/closing soon had that solved.
PS: We once had an old electric kettle in a shared common room, and one day I cleaned it out as it was full of gunk and limescale. After that, it leaked badly, and we had to get a new one. It was the gunk and limescale that was holding the kettle together....