This is for the most part the build I saw 1v5 the enemy team several times in a game and go 20/0. The only difference is the person I saw had 6 armor pen marks(rest ad) and flat magic resist glyphs.

Getting a BT GA is pretty much what you do if you gonna carry the game and all the other builds don't seem to consider this.

Definitely an underrated champion. I knew a guy who went all the way to 1800 primarily playing Riven.

Kennen and Nida solotop are played as AD not as AP usually, rumble ofc is an ap, and you will run some MR as soon as possible, but as first item i still prefer the cloth armor and 5 potions (actually more for the pots).
I said Riven is not a Tank, but need survavibility, and Randouin's Omen have some nice stats (and the awesome active effect).
Hope that helped.

The tanky items are situational like Randuin's Omen is better than Last Whisper if ur low on money

Buying armor when facing Magic damage tops doesn't seem that logical. Why not buy the MR from your boots instead?

Also you mention riven not being a tank; yet at the end of your guide you seem to mention a fair amount of tanky items for her to build. I found that rather confusing.