Would it be possible to mod infantry units so they can Firering while withdrawing, and artillery to 'fire by recoil'.

As it is now a unit which is under pressure can't be taken back a bit. If you want them to move to the rear your line unit will need to break formation and run back. This is bad! Units in this period where trained to fire, step behind the rear rank and reload. In this way you enlarge the 'killing field' between yourself and the enemy.

Similair artillery fired by recoil. A cannon had a heavy recoil witht he gun jumping back a few metres. The artillerists rolled the gun back into position and loaded. When the pressure grew too much and the enemy found their range they fired but didn't push the piece back into position before reloading. In so doing enlargging the space between them and their assailants. I am now wondering if cannon in ETW even have a recoil.

Anyways, can these two things be modded into the game for a bit more realism and tactical flexibility?

If you want an example try 'Sid Meier's Gettysburg'. In this game it is used already.