From my experience with boards:
1. if you want to use tags for other purposes than options are limited, one quite obvious is card name, e.g.
STORY: this is card name
2. this is definitely not possible and I don't have a good workaround to offer. I would just go with mix of the most relevant custom fields for all relevant card types
3. no dependencies strictly speaking, only some out of the box automations are available and furthermore if cards are connected with WF items than potentially you could explore Fusion to see how cards/objects could get connected.
good luck!