BJT how to explain in terms of conventional current flow?
Conventional current flow is nothing more than a mathematical convention for assigning voltage polarities across components in a circuit. Period. It is completely unrelated to the concepts of analyzing majority and minority charge carriers ("hole flow" and "electon flow") in materials. 

In your figure, blacken out the interior of the BJT so you do not know what's inside it or its theory of operation at the atomic lattice level. Now reverse the directions of all the current arrows, keep the voltage values and polarities the same, and bingo: you have the conventional current flow analysis of the circuit.