1. 그러나 꼭 엎드려 자야 한다면 쿠션 등을 얼굴에 받쳐 최대한 허리가 구부러지기 않게 해 주는 것이 좋다 (구부러지기 -> 구부러지지) => However, if you have to sleep on your stomach, it is best to place a cushion under your face to prevent bending of your back as much as possible. 최대한 is used as an adverb here working on the verb 구부러지지, not an adjective for 허리. 최대한 = 1. (n) the maximum possible; the greatest extent, 2. (adv) = 최대한으로 = as much as possible. 구부러지지 않게 = adverbial form of 구부러지지 않다 = do not bend. -지 않다 is the standard negation expression. 2. 둘 다 매운 거 먹고 똥꼬에 점화 붙었나봐. => Both of them might have their rear on fire from eating the hot stuff. 점화(ignition, lighting) and 붙다(to stick/attach/cling to something) are as you said. 점화's verb forms are 점화하다 (to ignite) and 점화되다(to be ignited - passive). 붙다 in 점화 붙다 is redundant, since 점 means to ignite, 화 a fire, and 붙다 again to ignite (불(이) 붙다 means "a fire ignites"). I've never heard 점화 붙다 before (it sounds dumb). 불이 붙었나 봐 would have sounds the most natural in that context - 점화 is not necessary. You can forget about 점화 붙다 and just remember 불(이) 붙다, 점화하다 and 점화되다.

1. it means you have to make your waist not curbed 'as much as possible' 2. First of all, you have to know that '점화' is derived from a kind of game, "league of legend'. it's one of skill names of the game. it is normally literary and only used in conversation among young boys and men (maybe under 30) anyway, it means ignition and the reason why they use the word is that koreans think pain that they feel after they eat very spicy food is similar to burning.