You're better off going for flat AP due the ap ratios on Cho's abilities are so good (this is assuming you're going for AP build). You will most likely be kinda squishy so you don't want to be in melee range.
For tanky Cho, Nashor just doesn't give necessary stats.
It can be. But usually you wanna do a sort of hybrid tank/AP for Cho. I have seen Nashor's Tooth used a few times by people who want to build Attack Speed/AP Cho, which can actually work if you can play it right.
So it all depends on how you want to play or what your team needs more of.
If you're looking to splash in a bit of damage to tanky Cho, wits end is a better splurge when ahead. If you're full AP, you need to burst people out, in either case, the answer is no.
For tanky Cho, Nashor just doesn't give necessary stats.
If you go tanky there aren't tanky stats on it that help cho so it's really awkward to buy
So it all depends on how you want to play or what your team needs more of.