In the Siddur of Rav Saadia Gaon on page 136, where he discusses the Pesach Seder, he says that when we do Orchatz we are supposed to make a bracha, and after Karpas to say Borei Nephashot. However, Ashkenazim and Sefaradim do either. If to do this was the norm then, why do Ashkenazim and Sefaradim deviate, and if what they are doing is correct, why does the Rasag deviate? And if there is an inconclusive machloket, what are rationales of each side?
As explained in Shulchan Aruch HaRav the majority opinion among Rishonim is to say a bracha on washing when dipping vegetables the way we are with Karpas. Tosfos כל שטיבולו disagrees, and therefore we don't say a blessing (as it is a safek). There is a common practice to rely on this opinion completely and not wash at all for dipped vegetables.
So in this one, Rav Saadia Gaon is well understood. The justification for the practice of not washing is a bit harder, but it has been discussed in Achronim (see e.g. the Magen Avraham on S.A. O.C. 158 s.q. 8).
Regarding the Borei Nefashos after Karpas, Shulchan Aruch HaRav explains that this opinion is part of a larger machlokes rishonim if the blessing of Borei Peri Ha'Adama on the Karpas is needed to include the Morror later or not - in other words is the Morror anyway exempt because it is considered part of the meal, or not.
According to the opinion that the Morror is part of the meal and does not require its own blessing of Borei Peri Ha'Adama, then the Borei Peri Ha'Adama on the Karpas is not connected to the Morror, so the Karpas requires its own, seperate Bracha Achrona - final blessing - and cannot be fulfilled with the blessing after the meal. This appears to be Rav Saadia Gaon's opinion.
Here again we are concerned with the other opinion, and thus not saying a blessing (at least) out of doubt. Although Rav Saadia Gaon clearly expected that a person would eat a Kezayis of Karpas (as this is the standard amount of eating), I don't know if he holds that this is a requirement. There is a practice to eat less than a Kezaiyis of the Karpas in order to not be required to say an after blessing according to all opinions.