"The first one is: is it better to realign (estimate & reslice) each session separately (thus, calculate one mean image/session) or to realign them all together (mean image/all sessions)?"
It's best to realign them all together. SPM "understands" the session structure. It realigns the first image of each session to the first image of the first session, and for a given session realigns the images there to the first image there.
"Second question: how does SPM deal with multiple sessions in the "Slice timing" step?"
It does slice timing correction on each session _separately_, which is correct.
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