Does prayer become invalid if one does not sit correctly—such as with tavarruk or iftirash—due to pain?

The preponderant view is to sit in the iftirash posture in all sittings of the prayer, except for the last tashahhud in a prayer with two tashahhuds, in which case it is done in the tawarruk posture. Violating this sunnah posture does not invalidate the prayer, but it causes one to miss the reward of the sunnah.