What kind of theory is this?
Ok, here is my theory:
The Ukrainians attack Snake Island by air, as they did before, and destroy various materiel, as they did before, including a helicopter. This is visible on video and we see the wrecking on the next day.
The Russians are embarrassed, because this shows that they are apparently incapable of keeping the Ukrainian air force from striking the island at will. The timing is awkward as well. So they make up a story about how ACTUALLY Russia won a great victory there, destroying many Ukrainian forces, great debacle! The Ukrainians are so cocky, but reality is not twitter, kokoko~ You know how they act in their propaganda broadcasts.
They find that NATO tube somewhere and find some corpses. Maybe it's their own dead from the attack, maybe it's living soldiers posing for the video. I think it's the latter, since none of them have obvious injuries. They film a shitty 10 second black and white video and blur out the faces, so nobody can identify anyone. However, because it's such an improvised rushed job, they accidently have one of the "corpses" wear a belt that is only sold in Russia and features a symbol the Ukrainian military doesn't use. So they delete the first release and re-release it with the entire bodies being blurred out.
They dig up the name of an Ukrainian officer who died around that time and claim actually he was killed in an attack on the island.
There, it fits all the evidence, it fits how both sides behave (Russians have been caught staging shit and lying about events many times during this conflict), it requires no secret battles that nobody noticed and that left no traces. It requires no bizarre motivations on the Ukraine side (a suicide attack that can't possibly succeed or accomplish anything an air strike can't accomplish just as well), or on the Russian side (letting the Ukrainian air force bomb a strategically located outpost to rubble and destroy your equipment there just to bait in some ground troops, even though you were actually capable of destroying the Ukrainian air assets used in the operation at basically any point).
You want your artillery as far away from fighting as you can without it losing effectiveness.
So if the Russians are pushed back 20 miles, that means the artillery can move up 20 miles without increasing the danger. Likewise Ukrainian targets move out of range of some Russian weapons and make attacks more risky.
The entire point of this push is exactly to try to force the Russians to move troops away from the Donbas and Izyum, but the Russians aren't biting so they still have to make a very risky river crossing if they want to put more pressure on them, their only choice is to pick where.
I don't know what their exact goals are or what the exact situation on the ground is, but I don't think it's as harmless as you make it out to be. If the region was truly useless, the Russians could have just left it, as they did in the north. I think there are things in Belgorod Oblast and Izyum they would rather keep the Ukrainian army away from, but they lack the ability to put up more of a fight than they currently do.
It's not like the Ukrainians are the only ones who suffered severe losses in this war.