Statikk shiv is a fairly good alternative to RFC; it gives you the crit you need and the attack speed to weave in plenty of aas between R's and abilities. I believe it is a very good item, although the range from RFC is slightly better.
You would buy Guardian Angel if you are ahead or if the game drags out. The base stats are good for Corki and the passive gives you a second chance in team fights, because remember; you are a carry champion and your death could well mean a lost fight or even game.

How Corki works is his basic attacks are permanently modified to deal 20% AD physical damage and 80% AD magic damage, even though, as we know auto attacks scale with AD. As well as that his abilities scale with mostly AD: Q - 50% AD, 50% AP; W - only 10% AP; E - 10% AD; R - 15/45/75% AD; 20% AP. This makes building him with AD items much stronger. His Q, W and R all do magic damage while his E does mixed.
The mistake you're making in your observations is assuming AD = physical and AP = magical, which although is true a lot of the time is not in this case. This difference is minor but quite impactful. Interestingly, I've seen many people build armor against a fed Corki due to the items he builds.
To sum up Corki scales mainly with AD (autos, q, e, r) but his damage is magical mainly because of his passive.