The City that Never Sleeps DLC is pretty much an extra scenario available after the game's launch, it won't just be a 1 mission thing - it'll be a few hours long. Will be available as a separate purchase of course.

If I'm not mistaken, the DLC is available if you purchase the Digital Deluxe edition. Of course, no physical copy for this.

There is still lots of challengers mission will be unlock until later part of the game. You just have to continue to play story. I got these challenges quite later part of the game. I think its around 65% -70% game completion section.

He's not just an easter egg

he is  

Who said you have to?

You just need to collect all gold if you want to unlock ALL the suits + mods + gadget upgrades. If you only care about the suits, you don't have to get all gold - only a few will be enough.

     

You can plat the game regardless. You don't have to save scum to do it. All task master challenges and base challenges can be re-done as many times as you like until you get the score you want on them.

Then I don't know what else to tell you... you're pretty much saying "hey I have a car but the mall is 30km away and it's a hassle to drive but I don't wanna walk either".