They made a reference that they would be attacking Cael-Um soil when attacking the Tower of Heaven, they made no reference that it would destroy the entire nation, and that is much larger than Fiore, they explicitly stated that Etherion is the only thing that can destroy a structure of such size. Planet's are not all made equal, besides Pluto is not a planet for a long time. I am not sure what you mean for the second part, but it is much more reasonable, with both statements in the manga, details depictions and the cacl to show would make it much more feasable to use than assuming something that we don't know is possible.