Was it naive? No, definitely not.
After Buffy's death, Willow justified bringing her back by saying that because her death was mystical, they actually could bring her back without the same consequences as those that would apply to someone like Joyce. Great, ok, makes sense. If Willow truly believed, (and had presumably done the research to back it up) that Buffy's death was truly mystical in nature; that it occurred on the way through the portal as it was being closed, then it would make sense that she was confident that Buffy truly was in a hell dimension, (namely Glory's "home.") Based on past experience, when a character dies in transit through a portal, they actually inhabit that portal.
Now, was it irresponsible? This time, there is a body. Willow would have had to make the logical leap that in this case, although Buffy's body didn't end up in the hell dimension, her essence separated itself and went without her. This didn't happen with Angel, or in the Wishverse, or in the demon underground factory in L.A. The rule as far as Willow knew had always been that when a being passes through a portal, it all goes together. So for Willow to assume that Buffy was in a hell dimension was irresponsible. Of course, Willow did freely admit that she didn't know, but that the possibility alone was so sickening that she couldn't think of what to do other than take action. And everyone else seemed to agree.
As the audience, I don't think we can assume that Willow has done the research to support the hypothesis that her essence could separate. We don't see her do it, and it doesn't come up in conversation. So it appears that more than anything else, Willow is desperate to have her best friend back.
Whether she was right or wrong is a matter of personal opinion, ultimately. But think about it in the context of the Buffyverse. These people are opening and closing portals and seals, bringing lives back and ending them, all of the time. Willow loves her friend and yes, selfishly--wants to bring her back. She has seen--for example--Angel spit back out of hell for no apparent reason, and from her perspective, why shouldn't Buffy have that luxury?
I also think we have to take into consideration that although Willow is the one who gets blamed, if Xander or even naysayers such as Giles and Spike had the power to do something about the possibility that Buffy was suffering torment in a hell dimension, who's to say that they eventually wouldn't have? I think the uncertainty would have been too much to bear for any of them, over time, given the ability to do something about it. I really do. But of course that's just opinion.
All that being said: is it wrong, what she did? Yes, in a very fundamental way. But it is more than understandable and I don't think it's a stretch to be able to sympathize.
