Is it safe to put the pc case above my extended mouse pad? (mouse pad: corsair mm300 pro)
Yes put the pc case above the mouse pad.Are you asking if you can put a PC on top of a mousepad? If so, yes.
Minor issue if you have vents at the bottom and the pad is thick enough to block them. And I suppose the weight will leave marks in the mousepad over time.
No it doesn't have intake fans below, only for psu with dust filter.if you have intake fans in the bottom it may lead to a bit of dust buildup around the pad but nothing dangerous to it.
if it's not hot enough to damage the desk, i'm sure the mouse pad will be fine.