Alright dude, i'm a diamond ADC here to help you with your questions and will tackle each in order:
- While it is true draven some of the strongest trades of any ADC, there are some other adcs that can actually trade back with him like kallista and miss fortune. Also, you have to consider what support they have against yours because that will determine who has the better trades. Example: You have a blitzcrank support and you're against graves nami. You won't win trades unless blitzcrank hooks someone.
And the true strength of draven is that he deals alot of dmg when he can maintain his spinning axes and its sustained dmg too so longer fights can easily favor him.
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When to build a defensive item?
You build a defensive item when you're unable to deal dmg to the enemy without taking major dmg or dying instantly or getting CC'd to death. When that happens, you need to immediately invest into a defensive item. Keep in mind that maybe you're positioning poorly or engaging at the wrong times that explain why you're taking so much dmg or dying in teamfights (if this is the case, you may not need a defensive item yet and should instead fix these individual playstyle issues). If you're extremely far behind in gold and do very minimal dmg in teamfights, do not bother building a defensive item. Instead, rush LATE GAME dmg items like infinity edge, phantom dancer, last whisperer. You always need to make sure that you're dealing decent dmg before you can even consider building into a defensive item. The defensive item's purpose is to allow you to stay alive long enough in teamfights to make an impact with your dmg (most of the time).
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Triforce on draven?
Its something I personally build for fun if I'm really fed. Otherwise, its completely unnecessary since draven scales better with flat AD items.
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Heal or Barrier?
Heal has more utility than barrier does. The only main benefits of barrier is the fact that it has a lower cooldown and the shield is higher than the heal. Even if u do not feel like you need heal, you can save allies with it. Aside from the mobility, heal can allow you to win certain 2v2 scenarios and other cases when you're fighting with another ally.
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Which defensive item to buy and when?
- Get it when you're getting heavily focused in teamfights to the point where the enemy can actually kill you OR if you want to play more ham in teamfights since getting this item allows you to front line in teamfights (as long as you have the passive up and the back-up of your teammates).
- One of the standard defensive items for ADCs, especially in competitive play. This item is great if the enemy has a few troubling abilities like a nocturne ult or blitzcrank grab (generally spells that have a pretty long cooldown once spent) that you have to watch out for.
- Get this if the enemy has snares or fears or hard CC that you can cleanse out of. You will need it especially for the CCs that have a longer duration and if the enemy has multiple CCs they can easily chain. This is item is also great as a 6th item if you're really fed since it provides both dmg and a defensive ability.
Last but not least, 
isn't a summoner spell you'll consider in the current meta since by not taking flash and heal/barrier for bot lane adc, you are a very easy tower dive and gank for the enemy. You could probably take it if u were doing top or mid lane draven alone.
. In my experience no one can trade with him. If you can keep a draven alive for at least 5 seconds in a team fight everyone will be dead or dying. What i need is some build help. Not with his core build but a defensive build and summoner spells. Essentially i dont know when i should build certain defensive items or continue getting damage. ( I cant play his
build yet so dont talk about that one.)
duh. But i usually run 
on draven. Due to the fact that i can keep his W off cooldown with his axes and my i build 
for trading. So with that i dont need the speed or health from 
. And i never use 
would be viable in certain situations.