My personal decision is:

because sivirs passive is just effective when she's moving
she has ~25% dodge left i know i pretty awesome but i would
also think to add a tanky build because of the dodge....

the playstyle would be like : around with having 50% dodge + armor + magic resist + life

Well, in case u mean doran's blade i think it could be highly beneficial for early and mid game play. But iam always a little bit conserend of getting items, which u can't upgrade.

1) I agree that Youmuu's Ghostblade is definetly better for melee chars in general, but the whole package it provides to Sivir is just awesome. 4seconds may not sound as long, but u have to remember that actually this item was choosen because of its armor pen, its cooldown reduction, its damage boost and that it is build by cheap parts (every forced trip back means u get back to the lane/game stronger) and one of this parts is even a god per second item! The active ability on yoummus is just an awesome addition, which completes Sivirs urge for Attack speed and its provided movement speed burst is yet another possibility to make use of it. Engaging, escaping and hunting.

I definetly agree with you, that in a 5v5 premade or high elo solo queue it could be a better idea to rush for hard damage, but in the context of my guide and with the thoughts behind it yoummus (in a combination of Shurelya's Battlesong) is just better!

2) I love the boomerang! But ricochet does more damage overall is, far cheaper (mana cost) and Ricochet isn't a skill shot. I will explain and defend my skilling order (although i think i need to change the skilling order in this guide to On The Hunt> Ricochet>[[Boomerang Blade
]]> spell shild, otherwise it could be missleading (which doesnt't mean i wouldn't in certain circumstances prefer shild over boomerang!).

3) No i shouldn't and i mustn't :)
If youre playstyle benefits highly from certain abilitys in the masteries, there is no reason to follow the trees till the end.. just because "you should".
If you however wanted to argue that that the 4% damage will cause you to be dead less then then i would have as a benefit by being on the battlefield back faster, i.... i need to think about it :)

Well i think Guides and specially the thought behind them should come as a package and try to cover everything :). The choices you make later are build upon the choices made earlier :)

I am actually considering changing up the early game order to incorporate more doran's, but I stand by the three solid points I made.

1)Youmuss' Ghostblade should be left to melee characters
2)Boomerang Blade should be focused
3)You should go a full 21 in one of the mastery trees.

Additionally, I did not even address the other items that you had because people build differently.

What sounds like constructive critisism, isn't actually! There is a reason why this guide is called "brainstorming" and the idea behind it and specially behind the items is completely not considered!

But ofc i'am guilty too to publish a guide which isn't fully ready, specially the chapters which i will add in near future will hopefully enlighten the dark ;)

However my guide is different from youre guide, because the mindsetting behind it is different.

You suggest to get Bloodthirster first (after/among with Berserker's Greaves) - this sums up to a total cost of a little bit less then 4k.
The Brutalizer, Ninja Tabi, Philosopher's Stone, Kindlegem u invest less gold then in Bloodthirster and youre overall benefits are just better, but more about this in the chapter about the advantage gap!

Sorry, but Youmuss' on a none melee character is just bad. You benefit very little from the active because you attacks are ranged not melee. Leave this to the melee characters. Also, early game is where Boomerang Blade actually has significant punch so you don't want to put that off. Plus, if you are going to work your way all the way down the defensive tree, than go full 21 and get the damage reduction over good hands, which means you expect to die a lot. I am not going to vote either way yet, but this guide needs a lot of work.