My reasoning behing attaining thornmail & FoN over Zhonyas and Abyssal Scepter is very direct and just a mere preference. I prefer thornmail in the case explained because if an enemy champion, such as Tyrn gets fed, then you want to get as much Armor as quickly as possible, not to mention the unique passive should deter him from attacking you first, if he's any good. The other under lying condition that I haven't stated explicitly is that at this point, Fizz should be fed as well, or at least has a killing spree under his belt. Also my other concern is that at that stage of the game the Hourglass may be a little too pricey to purchase as a flexible substitute in retaliation to one enemy champion. However if you later wanted to sell thornmail and purchase Zhonyas in the end game that seems very reasonable.

Now in terms of the FoN, this is a more debatable option since the items are so closely priced and in terms of stats. The one option where I definitely see your point is if you are struggling for kills, in that case Abyssal Scepter would be necessary since you wouldn't be able to purchase enough AP in a short enough time span. I was thinking about it from a viewpoint and so I will note you in my build when I make the appropriate changes.

wouldnt it be better to get zhonyas hourglass over a thornmail for ap, armor and a 3s immunity, and an abyssal scepter over FoN