Can what happened before the writing of the destinies in the Preserved Tablet be considered qadar (divine decree) or by the will of Allah?

The will is part of the levels of divine decree. The creatures that existed before the Pen and the Tablet were created by Allah through His knowledge, power, will, and volition. There is no evidence to prove or deny the inscription of those creatures. The Ahadith that mention Allah inscribed everything in the Remembrance (Al-Dhikr) refer to what He created in six days, not what was created before that.