invisibowl

It wasn't "found", rather built from smaller components (that were found, of course, and by various computer-aided search techniques plus manual finagling). Its final assembler released a decently-quick rundown in the form of a video recording: https://www.youtube.com/watch?v=on3ZLLKQp-4

It's actually easily provable, and simply by observation, because "smaller things combining" has a much simpler definition than what you're assuming! What it actually refers to is how some spaceships, such as the Waterbear -- https://www.youtube.com/watch?v=on3ZLLKQp-4 -- are constructed explicitly as a circuit of building blocks (as you can see in the video), whereas elementary spaceships like this knightship consist of "space dust" that isn't separable into independently-functioning components.

c/3 diagonal is actually impossible as well - the limit is c/4, proved by Conway himself in Life's early years. His proof can be found here: http://www.njohnston.ca/2009/10/spaceship-speed-limits-in-li...