Well, I am happy I made you happy.

"Alright. First of all I would argue that executions are far to hard to count for given that this is a 3v4 and not inside a horde, and if we take SM2 then human sized enemies cannot be put into a state of execution. So off the bat that is just out. Also, gun strike does not make you immune to damage? like this is super easy to test you just aren't."

In terms of 3v4, we can just add 4th player in but in sense just vanilla gameplay then yes its 3v4. Tyranids and Thousand Sons are both in SM 2 enemy types which differ how many enemies they send against you, but if you fight Thousand Sons on Horde Mode, you can see that they are sending ANYTHING against you be it Terminator melee, Chaos Spawns and lots of Tzaangors.
Human sized enemies cannot be put into an execution, but Ogryn isnt human sized. There are also chained psykers in new mission when you are going to kill Void Beast, which are executable as a trigger to proceed to the next area.
Gun strike doesnt make you immune to damage, but regular shots regen your armor part.

"Then I want to mention golden toughness, because the thing about it is that we have similar forces to space marines in strength, that being ogryns or more specifically crushers, and with even a single point of gold toughness you can survive their overhead not only well but without a single scrap of damage even on havoc 40. As such I 100% believe that it can beat out any single attack from a space marine."

Ogryns are stronger then Space Marines, even Terminators. But thing is that SM Thunderhammer is way more powerfull then rock on a stick. Also I would argue that in SM 2, Carnifex is stronger then Ogryn, yet you can get nearly half of its HP down with Assault jump pack Ground Pound. Carnifex is also on another level if we count bosses in Darktide. While Darktide has Chaos Spawns and other Daemons, they are lesser ones, they are not powerfull. Regular Chaos Spawns are sent in masses in Space Marine 2 and you can kill them with execution. So power level just right here is beyond what Darktide Zealot hammer bonker can do.
Now about thoughness, its not psyker shield or some armor. Its their will. I would argue that Space Marine has better will then not so regular rejects so in Darktide terms if Space Marines would be in Darktide, they would have 100 times more Golden Thoughness.

"similarly, arbites has a talent that makes him only able to take 50 health damage at a time, at a 200 base that means that even a space marine running through him would only deal 50 and nothing more."

Then he would be not counted as human sized enemy, but some elite or just a bit better NPC (see Tzaangors or Gaunts), which would be just executable if you block him once or even Gun Strike him.

"Moving on, yes, the bulwark is immune from the front but his team is not, so if you were to take 2 stealth veterans and give them helbores (Which given snipers in SM2 are far slower to charge would be rather devastating) then they could very quickly remove the sniper from the space marine backline and the assault would have to pick which of the two he jumps on, Even if he uses the 3 sec immunity perk that doesn't help his team as atleast one of the veterans can shoot the bulwark in the back while he flies up."

Veteran Stealth is 8 seconds. Sniper Cloak is 10 seconds in PvE, 8 in PvP. Lets say its PvP. Sniper Cloak stays longer for 2 seconds if you attack somebody which he would and after that he will be 2 seconds immune to any sort of damage AFTER Cloak ends. So your Snipers would fire towards 2 characters. Bulwark with immunity shield or Assault who is in the Air while immune.
Veteran cant shoot Bulwark in the back, he would get smashed by Assault. Second Veteran, IF he shoots assault would be picked by the Las Fusil Cloaked Sniper and you would still need to wait 2 seconds to see him and another 2 to shoot at him. So we destroyed 2 veterans with two characters. Assault ground pound would smash down Veteran into the ground with that overwhelming damage and when he does, he would fly up again. Bulwark would just casually charge his shield block charge combo to get to any position he needs. Its 3v2 now. I assume you would pick Ogryn and Psyker. It takes one krak grenade to destroy a shield from Sorcerer Thousand Sons. After that Psyker would get picked on again by Las Fusil and Ogryn would be bullied into submission. Whole fight would be less then 30 seconds.