Is it correct that I need Apple’s Lightning to USB Camera Adapter to connect any usb midi keyboard?
Yes
What´s the deal with midi sync? Does Ableton Link work or is there another way to sync my ipad to Ableton?
It’s up to the app to implement support (either sync or Ableton Link), most do, though less support being the slave for straight midi. Ableton has a big list of apps with Link support https://www.ableton.com/en/link/apps/
I use AUM for routing midi, AUM & AudioShare are really useful, especially if you have external gear.