1)Does a suppression marker goes with the unit(s) as they retreat, and apply in the hex where they end their retreat?
(Not sure about this one.)
Night Raids: 2) When counting the 18 MP night raid distance, is the trace to the target hex itself, as though the raiders were paying to enter the hex
16.4.1 says "do not include the cost to enter the target hex" and "simply total the terrain cost of all intervening hexes," so, no, you don't include the cost of entering the target hex.
3) In resolving a night raid combat, to leader modifiers, suppression modifiers and Soviet or Finnish morale mmodifiers apply as well as the shifts granted by the night raid table?
16.4 step 4 says "apply the column shift as indicated in step #3 and resolve the attack on the CRT with no other modifiers" (emphasis in the rules), so no, those other modifiers do not apply.