Lumon is the object, not teron, so why does teron have an -n?
This is the accusative of motion.
sur la tero = on the earth
sur la teron = onto the earth
See When to use the -n ending.
What's the etemology of "disvastigas"? Is the ultimate root "vasti"?
I got confused by this too. The ultimate root [also in English] is vast (i.e. huge). So vastigi corresponds to expand and thus disvastigi to spread.