EDIT: In Captain mode archers are kinda bad. If anyone read the OP and thought it was about Captain idk what to say.
Archery is very strong... as a result of shields and infantry being too weak.
Playing as archer, there's no exciting moment of skill expression where you get a triple headshot, for example. Other FPS or shooter classes in other games are based around these moments, and are what make them exciting and fun. The damage from archers is done over time, which stacks up to be very strong, but ultimately doesn't feel that rewarding for the archer.
Obviously, it doesn't feel good as infantry either because you can't effectively counter an archer. You can play around their shot timing, but that only goes so far when you can't actually block shots.
On your points:
1. I'm confused as to why arrow drop is your first factor. The bows DO have a drop, the game adjusts their aim. If the default angle didn't account for drop, people would just aim higher. I don't see how this has much impact. It is also much more intuitive this way that your shot goes to the center of the reticule.
2. Hard agree. This is the primary factor to me. Buff shields and archery would be much less oppressive.
3. I've never seen the damage values that you're describing. I've double headshot infantry before and they lived, just farther reinforcing the lack of skill because you should aim for body shots. Archers don't do damage based on single shots, they get it over time because it's so easy to shoot people with the way shields are. I would describe this as continuous vs burst damage.
4. I certainly agree with regular archers mounting any horse. Infantry and ranged mounting horses in general is a huge problem and feels very gimmicky. You can spawn in as a peasant and loot a shield+lance+horse and use a 90g class as a 170g one.
5. Crossbows unlike bows do have good burst damage. Still, buffing shields would make them much less valuable. I don't think they're stronger than the Palatine/Veteran/Khan'sGuard. They might do less damage overall as their DPS is much lower.