1. 5d is definitely unshakable. Yes freeze break is on during training mode.
2. You don't catch them in the air, your dash cancel reaches them as they touch the ground - the 5c connects effortlessly into jump cancel bs. You let them land, thats the idea and no they cant shake in time before 5c connects.
The whole combo works, tested it several times, every type of recovery is enabled.
Note: This is on Ragna. Jump canceling 5c into j.b > j.c > etc. on an standing opponent only works on characters with a tall enough hitbox.
Please, try it out and then get back with me. Several people I've spoken to about this seem to feel it has potential and I would like people to try it for themselves before passing judgment.