I’ve used the Whale chain tool for a few years and it couldn’t be easier.
Not cheap though as they’re about £85 now and annoyingly I can’t find mine.
You could go old school with two hammers or a G clamp and a ball bearing but that could easily end in tears.
I wouldn’t bother with pushing out pins, just cut the chain off with an angle grinder.
My one looks so exactly like that (except for now label on the box) that it may be the same one. Though I am sure I paid something like £25 based on a recommendation somewhere.
I also use it to split chains with no preparation and, well, it broke the chain. Tool was fine, though. Mind, I guess it makes a difference if you break are not breaking a master link.
If you are going to removes the heads off the rivets first then you may as well just do as Longshanks says and just cut the chain off. It was a lot less fiddly with the tool, though.