Q1: Yes, if are doing a mode-supe harmonic analysis, the modal analysis must include enough modes to capture sufficient modal mass (participation factors). The amount of captured modal mass (75%, 90%, etc.) will affect the accuracy.
Q3: I do not know what you mean by "matrix load in PSD datas". I would use a duty cycle, where each event represented acceleration in one direction. Defining and using duty cycles is described in the Introduction to Ansys nCode DesignLife training course. There are also several worked examples in the Ansys nCode DesignLife Help documentation.
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