Is a prostration of forgetfulness (sujud al-sahw) sufficient for not being able to perform some postures of prayer due to their difficulty?

There is no harm in sitting in prayer in any possible manner, and the prostration of forgetfulness (sujud al-sahw) is not required for an inability to adopt the sunnah posture of sitting. Al-Nawawi said: "No specific posture is mandated for sitting to be valid; rather, any manner suffices. However, the Sunnah is to sit in the tawarruk posture in the final part of the prayer and in the iftirash posture in all other sittings."