Translation:Hundred, thousand, and million are three numbers; million is the largest.
I agree. Also, I have just had to translate the sentence "Miliardo estas multe pli ol miliono" into English, and the given correct answer there is "A billion is much more than a million". Leaving the "a" out is just bad English. (I'm originally from Colchester in Essex).
Correct. Nulo (zero) and anything above and including million are treated as nouns with all the same rules. These are called substantive numbers. I find it an annoying inconsistency but I'd be interested to hear if anyone has a good explanation of what Zamenhof might have been thinking when he made this decision.
"The most" and "the least" use the same form of "plej". (Pli = more, malpli= less (collective noun like "water") or fewer (plural noun like "things"), not least.) To say "least" you would use plej+ the thing that is the least of, such as "la plej malvarma" = the "coldest" (which technically means "the most not hot"), or "la plej malalta" = "the lowest" (which is actually "the most +opposite of high"). Hope this helps.