Most of your questions can be answered with "it depends". I'll try to be more specific about when they're good or bad ideas though.
1) Building high health on Ahri might be a good idea when you're taking a lot of guaranteed damage; for example, against a very fed Karthus, it's impossible to avoid taking a lot of damage in teamfights from his ultimate. In most cases, Ahri's mobility is enough to keep her safe in teamfights. Zhonya's Hourglass is often a strong defensive item for situations where she'd require it, since it's harder to prevent autoattacks from AD Carries in lategame teamfights if your team is going on their backline, and you can use the clicky between cooldowns to win duels or survive while your team obliterates someone on you.
2. Avoid building Runaan's. It's extremely situational, and you should probably only consider it as a 6th item in very specific circumstances (needing to slow/kite multiple people for your team in the very late game, for example). A conventional start for most AD carries is 1 or 2 doran's blades early, possibly an early vampiric scepter, then either Bloodthirster or Infinity Edge. IE has more synergy with Ashe than BT due to her passive, but BT can be stronger if you need the sustain in teamfights. Static Shiv is very strong on her if you need wave clear (for defending turrets or pushing aggressively, like against split push comps), Phantom Dancer is stronger single target damage.
3. Stay away when you only have 3 minions and farm. He's extremely strong early and mid game, worthless late game as AP Yi, since he's entirely dependent on getting resets in teamfights. Once people are tanky in the lategame, he can't do anything. For Ashe it's trickier. Blade of the Ruined King is a good option because it makes it a lot easier to kite him - you can't slow him, but you can make yourself a lot faster. It'll often come down to your positioning (being too far back for him to alpha strike to unless he runs through your entire team, and hopefully they murder the hell out of him if he tries), and using your ultimate to disengage.
4. She can. People aren't entirely sure what to do with her yet. She's most conventionally played as an AD carry bottom with a support, but a lot of people have experimented with her in different lanes and roles.
5. The second blue buff is often donated from the jungler, around 7 minutes when it respawns if they started at blue, around 8 minutes if they started at red. If you don't have a jungler, whenever you can solo it safely without losing a massive amount of farm. I might wait for a while, like maybe around level 9-11 or so, depending on build.
6. Cackle and autoattack people to proc it. Red buff isn't bad on a support with a ranged autoattack, it just means it becomes a small peel in teamfights rather than a threat to the enemy carries.
7. Build more damage. I think this is more a consequence of you building static shiv and runaan's hurricane early. More single-target damage makes it far easier to get kills.
8. Sometimes yeah. Ideally other people ward for you, but you can't farm areas safely if they're unwarded, and you can't push for some objectives without wards. If that means you have to ward, then just do it. Carrying a pink specifically to do baron or dragon can often be a great idea if no one else thought of it.
9. Yes. It becomes pretty much a necessity to see when you can fight the enemy mid without dying to the jungler, and so that you can notify other lanes when your lane opponent roams. You often have the option of pushing aggressively or trying to follow them, and either can be valid options depending on how safe the lane they're roaming to will be.
10. Most people go aggressive in lower level play because it's fun, easy to remember when it pays off, and easy to blame your teams and forget about it when it doesn't. Mindless aggression is sort of the hallmark of bronze and silver play - people just going for kills when they can, and not knowing or caring if it'll backfire from having zero clue where the enemy jungler is, if the other person's build and summoner spells will mean they'll win, etc. The higher you go, the more people calculate about when their aggression is a good idea; going aggressive when they see the enemy jungler reveal himself elsewhere on the map, tracking enemy summoner spells, knowing at what level who is stronger and with what items, etc. Going aggressive is great if you'll win.