Maybe they already moved to Russia? How do you know the city still has 500,000?
There aren't reports (that I am aware of) about massive exodus from Mariupol. Like, for example, in Donetsk, Gorlivka or Luhansk who were besieged by Ukrainian forces, cut off from electricity, water, food and medical supplies, and put under the constant artillery barrage.
And why would the pro-ukrainians have more guns than pro-separatists, especially with so many separatists in the area?
There are many anti-Maidan activists, yes, but they are poorly armed. Ukrainian army is one of the best equipped European armies and their stocks are huge. Rebels are much stronger than they were in May or June, but still they are nowhere near the Kiev's forces.
By the way, what will happen to the thousands of pro-ukrainians who live in Mariupol if the separatists take it over? Will they be banished from the city? Arrested and tortured like other ukraine supporters?
I suppose they would become 'silent minority'. Except for some unfortunate but isolated accidents, there hasn't been anything like organized campaign aimed at the large-scale indimidation or ethnic cleansing of Ukrainians loyal to Kiev in neither DNR or LNR. On the other side... well, let's just remember that -according to UN- almost
900.000 Ukrainian citizens are refugees in Russia, that's about 80-85% of all people in Ukraine who were forced to leave their homes and run for safety.