Eidolon Synthsis 
From what I can tell this is an error in the fact that the builder does not give you an Eidolon as a synthesis it mearly runs a seperate overlay for that part of the character mods, that the class power "cSumFusEid" not "cEidolon" since at 16th level it then adds "cEidolon" as a class feature. This is just a guess. This happens for any feat requiring an Eidolon. I have tried to make a fix for it but since I am missing one key structure to figure it out, when you add a point to "hero.childfound[cEidolon].minion.child[resRaceCus].field[resMax].value" it does not also add to "hero.childfound[cSumFusEid].minion.child[resRaceCus].field[resLeft].value". I'm am sure that I'm off here because, I am not really sure if "hero.childfound[cSumFusEid].minion.child[resRaceCus].field[resLeft].value" is correct since it does not work when I make a Test Feat to try is. Though I can fix the red out for the requirements pretty easy. I am also not sure on how the interaction will affect the other arctypes. I think that most things that add to eidolon can be added to a synthsis in this way by just knowing the right location to add it in at.
Reference for Interactions on Synthsis and regular Eidolons found:
http://paizo.com/paizo/faq/v5748nruor1fz#v5748eaic9o7n
http://livingpf.wikia.com/wiki/Synthesist
Quote from Sean K Reynolds,
Feat Qualification: Does the fused eidolon qualify as an "eidolon class ability" for feat qualifications? Yes. Since the Ultimate Magic and Ultimate Combat books do not contain a feat that requires a "fused eidolon", the fused eidolon qualifies as a normal eidolon for feat selection.
—LPF Judge Vote Aug 28, 2011
Then later, a confirmation:
From the archetype:
"In all other cases, this ability functions as the summoner's normal eidolon ability..."
—Sean K Reynolds, Nov 11, 2011

Summoner (Synthesist archetype)
Extra Evolution Feat
- This is currently grayed out
- If selected, it does not increase the Fused Eidolon's Evolution Pool
- If selected, it generates the error message: Eidolon Required
Half-Elf Synthesist Favored Class bonus (+1/4 Evolution Pool)
- If selected (for any multiple of 4 times), this dos not increase the Fused Eidolon's Evolution Pool
I didn't think anything about either of those combinations, until one of my players showed me this post on the Paizo Blog where designer Sean K Reynolds points toward the line in the archetype description that reads: "In all other cases, this ability functions as the summoner's normal eidolon ability..."
From what I can tell this is an error in the fact that the builder does not give you an Eidolon as a synthesis it mearly runs a seperate overlay for that part of the character mods, that the class power "cSumFusEid" not "cEidolon" since at 16th level it then adds "cEidolon" as a class feature. This is just a guess. This happens for any feat requiring an Eidolon. I have tried to make a fix for it but since I am missing one key structure to figure it out, when you add a point to "hero.childfound[cEidolon].minion.child[resRaceCus].field[resMax].value" it does not also add to "hero.childfound[cSumFusEid].minion.child[resRaceCus].field[resLeft].value". I'm am sure that I'm off here because, I am not really sure if "hero.childfound[cSumFusEid].minion.child[resRaceCus].field[resLeft].value" is correct since it does not work when I make a Test Feat to try is. Though I can fix the red out for the requirements pretty easy. I am also not sure on how the interaction will affect the other arctypes. I think that most things that add to eidolon can be added to a synthsis in this way by just knowing the right location to add it in at.
Reference for Interactions on Synthsis and regular Eidolons found:
http://paizo.com/paizo/faq/v5748nruor1fz#v5748eaic9o7n
http://livingpf.wikia.com/wiki/Synthesist
Quote from Sean K Reynolds,
Feat Qualification: Does the fused eidolon qualify as an "eidolon class ability" for feat qualifications? Yes. Since the Ultimate Magic and Ultimate Combat books do not contain a feat that requires a "fused eidolon", the fused eidolon qualifies as a normal eidolon for feat selection.
—LPF Judge Vote Aug 28, 2011
Then later, a confirmation:
From the archetype:
"In all other cases, this ability functions as the summoner's normal eidolon ability..."
—Sean K Reynolds, Nov 11, 2011