I mean, it _will_ fail at stopping global warming, there's no "may" about it. It will probably have other positive effects though.

I'm very onboard for any potential fusion power generation, I just don't think it has any hope of saving us from global warming.

Can we use it to put the CO2 back in the ground?

I guess there are some irreversible effects once warming reaches a certain threshold however.

Yes, this is part of what would need to happen: using a super abundance of essentially carbon-free energy to do geo-engineering on a massive scale (including artificial carbon sequestration).