Hi there, yes electrocute can work, its good for burst combos, however comet and aery is more consistent output damage. I do like luden's Echo on support Neeko or top Neeko, but I think Protobelt is more optimal for making/setting up plays with your ult. You can run it however if you prefer it and it makes you play better though.

Syndra is an interesting one, it all comes down to positioning and being able to dodge the q's in the early game. Personally I like to take comet into control mages and just out trade her. If you do struggle against syndra, i would focus on shoving in the waves and roaming with your jungler or to lanes. Thats what i do. Just make sure to get wards, as you know, one E from syndra and her full comobo turn Neeko into a ghost. If you are struggling, MR boots and banchees sencond would be a good option. Everfrost is a nice item for mana plus cc, so helping your team out will benifit you in the mid/late game. Just shove and roam and repeat. If you know you wont kill her, i would take TP so you can get good resets :) Hope this helped a lil, im not a fan of most match ups in the mid lane atm.

yeah so this would normally be a 5th if there adc is fed, or frozen heart. normally if vsing like a kayne or zed. If you wanted to do more damage, a phantom dancer or static shiv could be beneficial. If per say, you were vsing an Ornn or Sion, someone heavy tank, a Lord Dorms would be good to add to the build. Hope I answered your question, if you have any more, feel free to ask :)

-Plz Buff Neeko