That is a massive assumption and not attractive, but fortunately it has little to do with the question. So:

"Could a large volcano (or any other natural means such as storm pressures or atmospheric disruptions), have sufficient energy and force to eject material from our planet"

Volcanoes are not well designed for efficiently ejecting small items at really high speeds; they do better at scattering large volumes of material into tropospheric or low-to-mid-stratospheric altitudes.  It would take a sizable coincidence (some particle or other catching just the right splash just right), but conceivably it could happen. A less comfortable problem is how happily life could hitch-hike a ride. Most of the stuff that our volcanoes throw particularly high is volcanic, mainly fresh volcanic at that. Recently molten in fact! The chances of anything alive being on anything that a volcano throws more than a few hundred metres, are pretty poor.

"(no matter how improbable)?"

If you are willing to settle for indefinite improbability, then why hesitate? Why not just posit quantum tunneling?  A blue whale and its surrounding bubble of water, plus enough water could tunnel into space, and by coincidence the fluids don't escape from around it. It might take a bit of a coincidence, but that is just a matter of improbability, not impossibility. If you claimed to have evidence that it actually had happened, but refused to release your data because they were proprietory, and the people requesting the figures were looking for something wrong with them, who could complain?

"Could Earth be a staging post in life's spread..."

For sure!  It wouldn't even require an improbable volcano. An obligingly-aimed comet impact could do the trick. On Earth we already have found material from Mars and Luna that seems to have been splashed by impacts, and that might have had a fairly soft ride. To do the same thing with bits of our own planetary crust would need a bigger bang, but nothing as challenging as tunneling a whale. For all we know, our most recent dino-killer might already have done the trick.

If you are interested in such matters, try an Internet search.  Use "panspermia" as a search term. It is a popular subject.  If you find any sites mentioning "Richfield" ignore those; he seems to be a know-it-all-sceptic who rains on panspermian parades. Doesn't swallow blue whales at all, nor even potted geraniums. He even coughs at bacteria!

Happy propagation!

Jon