I'd be extremely surprised if Wagner did it to themselves. They aren't a massively large group and if they're pulling this stunt going back into Russia, I highly doubt they want to lose any more resources going into that. They can't really afford to just cut into their resources like that like Russia could do without less of an impact. Plus, if word ever got out within the group that Prigozhin ordered the bombing of his own group, he'd probably be dead in a heartbeat. Prigozhin knows his only chance of survival is probably going to be reliant on his own men so I don't think that's a risk he'd be willing to take.
I think that Russia knows they are going to lose and knew that they were likely going to use the Wagner group so they probably thought their best bet was to try and eliminate then, use whatever excuse/scapegoat they want for doing it, and then just turn around and blame the Wagner group for what happened in Ukraine. Fortunately/unfortunately, Russia is inept and failed at taking them out and now we have an army marching towards Moscow!
I guess they had talked about doing this movie awhile back but it wasn't really possible at the time with everything going on with the rights to movies/characters. They never really thought this would have been possible, but once Marvel took control over everything, it became a possibility and Ryan went to Hugh with the idea. He was a little reticent on doing it because he thought Logan was the perfect ending for character, but they said that it wouldn't alter the timeline of what happened in the movie so he agreed to do it.
I guess Reynolds and Jackman are buddies too which probably made the convincing a lot easier.
That and the dump truck full of cash that @winebaths referenced probably didn't hurt.