Does the prayer become void by sitting between the two prostrations in the easiest way for one who cannot sit correctly due to pain and discomfort?

The manner of sitting in prayer is a Sunnah (recommended practice), not an obligatory duty (fard), whether it is the sitting between the two prostrations or for the two tashahhuds. Therefore, if the worshiper is unable to perform the recommended manner (either iftirash or tawarruk), there is no blame upon him for sitting in a way that suits his condition, and his prayer remains valid. Imam An-Nawawi said: "There is no specific posture for sitting in these positions that is mandatory for validity; rather, however one sits, it is valid, whether one performs tawarruk, or iftirash, or extends his legs, or raises his knees, or one of them, or anything else. However, the Sunnah is tawarruk at the end of the prayer, and iftirash in other instances." So, you may sit in the manner that suits you, and your prayer will be valid.