Calamity, Galloping Inferno will make copies of the cards exactly as they're printed, so animation effects won't be copies. However, since the copies will still have the original mana value, Bello, Bard of the Brambles's ability will apply to them making them enter as creatures. Since they are entering as creatures there's nothing keeping them from entering attacking. If you did not control Bello when Calamity's ability resolved you could not choose to make copies of City on Fire because you must choose a creature that saddled it, and City on Fire would be a noncreature enchantment that saddled it. If City on Fire were animated by a different effect like One with the Stars you would still be able to chose it, but the tokens would enter tapped and not attacking (since they wouldn't be creatures in that scenario).

TL;DR - Yes, the copies will enter tapped and attacking due to the animation effect being a static ability that will apply to the tokens rather than a 1-shot effect only effecting specific permanents.

The copy would untap all creatures you control that attacked this turn, but unless you're tapping them before the original resolves, that won't do much. If it's your main phase, it will create an additional combat step followed by an additional main phase, just like the original will, so you're turn structure will be that the next phase is combat, then main, then combat again, then main again. This will create an issue though, any creatures you attack with in your next combat will be tapped (unless they have vigilance), so they won't be able to attack during the following combat unless you can somehow untap them again. You will have a main phase between the combats to cast sorceries that can untap the creatures though.

When mimeoplasm says "exiled by it" it means specifically those exiled by another ability printed on the card, specifically it's first ability. This is because the two abilities are what are known as "linked abilities." Granting it a different ability such as those of Oona, Queen of the Fae or Shauku, Endbringer won't link the abilities and won't allow you to target cards exiled with those abilities.

607.2a If an object has an activated or triggered ability printed on it that instructs a player to exile one or more cards and an ability printed on it that refers either to “the exiled cards” or to cards "exiled with [this object]," these abilities are linked. The second ability refers only to cards in the exile zone that were put there as a result of an instruction to exile them in the first ability.

My apologies, I failed to mouse over the card and was thinking Starstorm. Which as that's an instant I should have realized was incorrect with the mention of giving it flash.

Because the ability does look at what the toughness was when the creature was last on the battlefield, rather than when it was activated, this line would avoid having you mill if you also had a Soul-Scar Mage in play, since the creatures would receive -1/-1 counters instead of damage and die due to having 0 or less toughness.