mounted upside down? lol thats a new one on me! are you sure it would be ok? ive kept cories and no issues with using a filter normally .. with that turnover im sure it would be fine normal
inlet slits at the top and the powerhead at the bottom... it will work fair enough and perhaps better as gravity will make the dirt fall past the filter
nah... on its side as i have a interpet pf1 to go at the top (380l/h) it will provide good circulation for the top and the fluval3plus amazing circulation for the bottom...