Hi Thank you for the Questions!
You are correct, any model can move or run into base contact with another and not receive a charge bonus.
If a model has been engaged by an enemy it cannot then activate. This is in the errata on the rules page and I am updating the books over Christmas break so this will be clarified in there for books purchased in 2025.
When no wounds are inflicted, the losing model withdraws back 1" away from the winning model. If it cannot move back due to terrain or other models then please see the "desperate" rule.
Yes after the charge has been declared, all combats are treated as individual fights so models will be pushed back across both sides depending on which individual model is the winner.
For swords, the modifier is relevant for whichever unit type it specifies gets it. I have re-written this for the Christmas update.
Yes the armour differences for the most part are variations of inexperienced (Defence 2) Experienced (Defence 3) Combat Hero (Defence 4) for both the basic army builder in the rulebook and factions in the army books although the army books do have more variations among units. For example, the Irish faction in the Dark Age book has the 'Wolf Warrior', a lightly armored fighter but still has Defence of 3 to represent that he is a tougher warrior although cannot equip a shield. If you wanted to have more accurate representation of your warriors, you can use the custom faction builder in the Armies Of The Dark Age book and include another unit's stats that you feel works for your army.
I did originally design the game to represent different specific armours such as leather, gambesons, mail etc. however it became difficult to represent all of their armour values and have them be beaten using a D6. And it also became confusing when playing against someone else where they have models that look identical (both in mail for example) but are two different unit types. So it became clear that I had to just have two main troop types with further faction specific units to avoid confusion between players and keep the game fast paced.
I hope these answers have cleared up a few things and if you have any other questions please don't hesitate to ask!
Many Thanks
Matt