I agree on most of your points. The only one I saw problems with was this one.
Last whisper: I'm sure you know the drill by now, attack speed isn't that good overall.The 40% armour pen is pretty good...so why is it so situational? It works like this Their base reduction-flat armor pen and THEN wits end and it's % pen is applied so if they had 50 armor(in this theoritcal case I'll ignore the in game system for a simplified version of 1 armor=1% reduction). If they have 50% reduction and then you had 20 armour pen you'd bring it down to 30%(this is NOT the in game values, but is a symplified version to help you understand it). then the last whisper would takee out 40% of their remaining armor reduction so it would leave them at 12% armor reduction. Since the way the system works puts the flat armor pen above the percent reduction it makes last whisper less effective. So against tank heavy teams it's good, but against a regular team I can't recomend it. If your a heavy tower pusher then yes it can be good(and is). This is NOT a bad item, it just isn't the go to item for most situations.
"Attack speed isn't that good overall"? You need some attack speed. It's as important to have attack speed as it is to have damage or pen; you'll want a variety of stats to make the gold spent cost efficient since many have diminishing returns. & you might not get such-and-such amount of seconds of freely hitting the enemy champ, but let's face it, it's not like you're getting kills without CC anyhow.
Your statements regarding armor penetration are untrue. % penetration is calculated before flat amounts.
The following is taken from the stickied post "Frequently Asked Questions on Mechanics."
Originally Posted by Stumpster
Q: What order is armor penetration/MR penetration applied in? (Credit to TiberiusAudley)
A: From top to bottom
% Based Armor/MR Penetration
Flat/Linear Armor/MR Penetration
Reduction and penetration aren't the same thing. You get reduction from things like Stark's and you get penetration from things like runes and Brutalizer. For its price, Last Whisper is one of the best DPS items throughout
the game whether anyone is stacking armor or not (though it should be noted that you can't penetrate armor past 0).
Edit: I'm with Leohan; it definitely makes sense for you to give the numbers on item pricing.