You know how heavily the skirmishers are used in the Feudal age, right? The 100 food would still worth paying for the double effect immediately after the Fletching as it is even cheaper than 2 skirmishers. Not to mention there absolutely would be other units like spears, archers or scouts and not make them alone. The archers usually have the armors late but here they would have soon, and here the scouts only could deal 3 damage per attack to the archers (as the melee upgrade also gets researched pretty late usually) while it’s 5 originally. The additional armor for the skirmishers (and archers of course) would bring a snowball.
Not just well but very well. After the Bodkin Arrow, the second archer armor would become worth researching as soon as possible too. The crossbowmen with this bonus would have as much armor as the knights with the second cavalry armor. It’s like the archers receive -2 damage from any units and buildings (but it feels more like -3 since the second archer armor usually gets researched late for most of civs). For comparison, the Persian bonus that Knights have +2 attack against archers is considered a pretty good bonus, and the double armor for archers is equivalent to canceling out a power greater than this bonus. Even the skirmishers could still have a trade with them, the -2 damage still make the thing different.
Exactly the infantry are not only slow and but also have no range so that they can have double effect armor in the early game.
Different modes can look different. Maybe they could be propped up and aim higher in this mode.