multiplexed row/col is not problem here. By adding more components (1 normal diode per key) you can detect all pressed keys.
Normal keyboard can detect even 4 keys (not all combination ofc) by clever design.
Yes, but that's usually limited to modifier keys. This app expects the user to use multi-letter key combos, which the keyboards usually don't support. I have several normal keyboards at home and none can deal with that.