Why is Illya able to survive firing Excalibur when Heaven's Feel Shirou couldn't? It's because unlike him, she actually has the prana reserves to use it! Their differences in magical energy spell the contrast between an effective Wave Motion Gun and a Suicide Attack. This fact is actually explained by Archer in Unlimited Blade Works.
Why does this Illya differ so much from the character from Fate/SN? Because she is a surrogate, while Chloe/Kuro is the suppressed original. Kuro is ruthless, calculating, and a powerful mage. Just like the little girl who summoned Berserker during fifth Grail War and controlled him flawlessly. No wonder she reacted violently when Illya called her a second-hand clone.